Gomega is a matcher/assertion library. It is best paired with the Ginkgo BDD test framework, but can be adapted for use in other contexts too.

Support Policy

Gomega provides support for versions of Go that are noted by the Go release policy i.e. N and N-1 major versions.

Getting Gomega

Just go get it:

    $ go get github.com/onsi/gomega/...

Getting Gomega as needed

Instead of getting all of Gomega and it’s dependency tree, you can use the go command to get the dependencies as needed.

For example, import gomega in your test code:

    import "github.com/onsi/gomega"

Use go get -t to retrieve the packages referenced in your test code:

    $ cd /path/to/my/app
    $ go get -t ./...

Using Gomega with Ginkgo

When a Gomega assertion fails, Gomega calls a GomegaFailHandler. This is a function that you must provide using gomega.RegisterFailHandler().

If you’re using Ginkgo, all you need to do is:

    gomega.RegisterFailHandler(ginkgo.Fail)

before you start your test suite.

If you use the ginkgo CLI to ginkgo bootstrap a test suite, this hookup will be automatically generated for you.

GomegaFailHandler is defined in the types subpackage.

Using Gomega with Golang’s XUnit-style Tests

Though Gomega is tailored to work best with Ginkgo it is easy to use Gomega with Golang’s XUnit style tests. Here’s how:

To use Gomega with Golang’s XUnit style tests:

    func TestFarmHasCow(t *testing.T) {
        g := NewGomegaWithT(t)

        f := farm.New([]string{"Cow", "Horse"})
        g.Expect(f.HasCow()).To(BeTrue(), "Farm should have cow")
    }

NewGomegaWithT(t) wraps a *testing.T and returns a struct that supports Expect, Eventually, and Consistently.

Making Assertions

Gomega provides two notations for making assertions. These notations are functionally equivalent and their differences are purely aesthetic.

• When you use the Ω notation, your assertions look like this:

        Ω(ACTUAL).Should(Equal(EXPECTED))
        Ω(ACTUAL).ShouldNot(Equal(EXPECTED))

• When you use the Expect notation, your assertions look like this:

        Expect(ACTUAL).To(Equal(EXPECTED))
        Expect(ACTUAL).NotTo(Equal(EXPECTED))
        Expect(ACTUAL).ToNot(Equal(EXPECTED))

On OS X the Ω character should be easy to type, it is usually just option-z: ⌥z.

On the left hand side, you can pass anything you want in to Ω and Expect for ACTUAL. On the right hand side you must pass an object that satisfies the GomegaMatcher interface. Gomega’s matchers (e.g. Equal(EXPECTED)) are simply functions that create and initialize an appropriate GomegaMatcher object.

Note that Should and To are just syntactic sugar and are functionally identical. Same is the case for ToNot and NotTo.

The GomegaMatcher interface is pretty simple and is discussed in the custom matchers section. It is defined in the types subpackage.

Handling Errors

It is a common pattern, in Golang, for functions and methods to return two things - a value and an error. For example:

    func DoSomethingHard() (string, error) {
        ...
    }

To assert on the return value of such a method you might write a test that looks like this:

    result, err := DoSomethingHard()
    Ω(err).ShouldNot(HaveOccurred())
    Ω(result).Should(Equal("foo"))

Gomega streamlines this very common use case. Both Ω and Expect accept multiple arguments. The first argument is passed to the matcher, and the match only succeeds if all subsequent arguments are nil or zero-valued. With this, we can rewrite the above example as:

    Ω(DoSomethingHard()).Should(Equal("foo"))

This will only pass if the return value of DoSomethingHard() is ("foo", nil).

Additionally, if you call a function with a single error return value you can use the Succeed matcher to assert the function has returned without error. So for a function of the form:

    func DoSomethingSimple() error {
        ...
    }

You can either write:

    err := DoSomethingSimple()
    Ω(err).ShouldNot(HaveOccurred())

Or you can write:

    Ω(DoSomethingSimple()).Should(Succeed())

You should not use a function with multiple return values (like DoSomethingHard) with Succeed. Matchers are only passed the first value provided to Ω/Expect, the subsequent arguments are handled by Ω and Expect as outlined above. As a result of this behavior Ω(DoSomethingHard()).ShouldNot(Succeed()) would never pass.

Assertions about errors on functions with multiple return values can be made as follows (and in a lazy way when not asserting that all other return values are zero values):

    _, _, _, err := MUltipleReturnValuesFunc()
    Ω(err).Should(HaveOccurred())

Alternatively, such error assertions on multi return value functions can be simplified by chaining Error to Ω and Expect. Doing so will additionally automatically assert that all return values, except for the trailing error return value, are in fact zero values:

    Ω(MultipleReturnValuesFunc()).Error().Should(HaveOccurred())

Similar, asserting that no error occured is supported, too (where the other return values are allowed to take on any value):

    Ω(MultipleReturnValuesFunc()).Error().ShouldNot(HaveOccured())

Annotating Assertions

You can annotate any assertion by passing either a format string (and optional inputs to format) or a function of type func() string after the GomegaMatcher:

    Ω(ACTUAL).Should(Equal(EXPECTED), "My annotation %d", foo)
    Ω(ACTUAL).ShouldNot(Equal(EXPECTED), "My annotation %d", foo)
    Expect(ACTUAL).To(Equal(EXPECTED), "My annotation %d", foo)
    Expect(ACTUAL).NotTo(Equal(EXPECTED), "My annotation %d", foo)
    Expect(ACTUAL).ToNot(Equal(EXPECTED), "My annotation %d", foo)
    Expect(ACTUAL).To(Equal(EXPECTED), func() string { return "My annotation" })

If you pass a format string, the format string and inputs will be passed to fmt.Sprintf(...). If you instead pass a function, the function will be lazily evaluated if the assertion fails. In both cases, if the assertion fails, Gomega will print your annotation alongside its standard failure message.

This is useful in cases where the standard failure message lacks context. For example, if the following assertion fails:

    Ω(SprocketsAreLeaky()).Should(BeFalse())

Gomega will output:

    Expected
      <bool>: true
    to be false

But this assertion:

    Ω(SprocketsAreLeaky()).Should(BeFalse(), "Sprockets shouldn't leak")

Will offer the more helpful output:

    Sprockets shouldn't leak
    Expected
      <bool>: true
    to be false

Adjusting Output

When a failure occurs, Gomega prints out a recursive description of the objects involved in the failed assertion. This output can be very verbose, but Gomega’s philosophy is to give as much output as possible to aid in identifying the root cause of a test failure.

These recursive object renditions are performed by the format subpackage. format provides some globally adjustable settings to tune Gomega’s output:

• format.MaxLength = 4000: Gomega will recursively traverse nested data structures as it produces output. If the length of this string representation is more than MaxLength, it will be truncated to MaxLength. To disable this behavior, set the MaxLength to 0.
• format.MaxDepth = 10: Gomega will recursively traverse nested data structures as it produces output. By default the maximum depth of this recursion is set to 10 you can adjust this to see deeper or shallower representations of objects.
• Implementing format.GomegaStringer: If GomegaStringer interface is implemented on an object, Gomega will call GomegaString for an object’s string representation. This is regardless of the format.UseStringerRepresentation value. Best practice to implement this interface is to implement it in a helper test file (e.g. helper_test.go) to avoid leaking it to your package’s exported API.
• format.UseStringerRepresentation = false: Gomega does not call String or GoString on objects that satisfy the Stringer and GoStringer interfaces. Oftentimes such representations, while more human readable, do not contain all the relevant information associated with an object thereby making it harder to understand why a test might be failing. If you’d rather see the output of String or GoString set this property to true.

For a tricky example of why format.UseStringerRepresentation = false is your friend, check out issue #37.

• format.PrintContextObjects = false: Gomega by default will not print the content of objects satisfying the context.Context interface, due to too much output. If you want to enable displaying that content, set this property to true.

If you want to use Gomega’s recursive object description in your own code you can call into the format package directly:

    fmt.Println(format.Object(theThingYouWantToPrint, 1))

• format.TruncatedDiff = true: Gomega will truncate long strings and only show where they differ. You can set this to false if you want to see the full strings.

Making Asynchronous Assertions

Gomega has support for making asynchronous assertions. There are two functions that provide this support: Eventually and Consistently.

Eventually

Eventually checks that an assertion eventually passes. Eventually blocks when called and attempts an assertion periodically until it passes or a timeout occurs. Both the timeout and polling interval are configurable as optional arguments:

    Eventually(ACTUAL, (TIMEOUT), (POLLING_INTERVAL)).Should(MATCHER)

The first optional argument is the timeout (which defaults to 1s), the second is the polling interval (which defaults to 10ms). Both intervals can be specified as time.Duration, parsable duration strings (e.g. “100ms”) or float64 (in which case they are interpreted as seconds).

As with synchronous assertions, you can annotate asynchronous assertions by passing either a format string and optional inputs or a function of type func() string after the GomegaMatcher.

Alternatively, the timeout and polling interval can also be specified by chaining WithTimeout and WithPolling to Eventually:

    Eventually(ACTUAL).WithTimeout(TIMEOUT).WithPolling(POLLING_INTERVAL).Should(MATCHER)

Eventually works with any Gomega compatible matcher and supports making assertions against three categories of ACTUAL value:

Category 1: Making Eventually assertions on values

There are several examples of values that can change over time. These can be passed in to Eventually and will be passed to the matcher repeatedly until a match occurs. For example:

    c := make(chan bool)
    go DoStuff(c)
    Eventually(c, "50ms").Should(BeClosed())

will poll the channel repeatedly until it is closed. In this example Eventually will block until either the specified timeout of 50ms has elapsed or the channel is closed, whichever comes first.

Several Gomega libraries allow you to use Eventually in this way. For example, the gomega/gexec package allows you to block until a *gexec.Session exits successfuly via:

    Eventually(session).Should(gexec.Exit(0))

And the gomega/gbytes package allows you to monitor a streaming *gbytes.Buffer until a given string is seen:

    Eventually(buffer).Should(gbytes.Say("hello there"))

In these examples, both session and buffer are designed to be thread-safe when polled by the Exit and Say matchers. This is not true in general of most raw values, so while it is tempting to do something like:

    // THIS IS NOT THREAD-SAFE
    var s *string
    go mutateStringEventually(s)
    Eventually(s).Should(Equal("I've changed"))

this will trigger Go’s race detector as the goroutine polling via Eventually will race over the value of s with the goroutine mutating the string.

Similarly, something like Eventually(slice).Should(HaveLen(N)) probably won’t do what you think it should – Eventually will be passed a pointer to the slice, yes, but if the slice is being appended to (as in: slice = append(slice, ...)) Go will generate a new pointer and the pointer passed to Eventually will not contain the new elements.

In both cases you should always pass Eventually a function that, when polled, returns the latest value of the object in question in a thread-safe way.

Category 2: Making Eventually assertions on functions

Eventually can be passed functions that take no arguments and return at least one value. When configured this way, Eventually will poll the function repeatedly and pass the first returned value to the matcher.

For example:

    Eventually(func() int {
        return client.FetchCount()
    }).Should(BeNumerically(">=", 17))

will repeatedly poll client.FetchCount until the BeNumerically matcher is satisfied.

Note that this example could have been written as Eventually(client.FetchCount).Should(BeNumerically(">=", 17))

If multple values are returned by the function, Eventually will pass the first value to the matcher and require that all others are zero-valued. This allows you to pass Eventually a function that returns a value and an error - a common pattern in Go.

For example, consider a method that returns a value and an error:

    func FetchFromDB() (string, error)

Then

    Eventually(FetchFromDB).Should(Equal("got it"))

will pass only if and when the returned error is nil and the returned string satisfies the matcher.

It is important to note that the function passed into Eventually is invoked synchronously when polled. Eventually does not (in fact, it cannot) kill the function if it takes longer to return than Eventually’s configured timeout. You should design your functions with this in mind.

Category 3: Making assertions in the function passed into Eventually

When testing complex systems it can be valuable to assert that a set of assertions passes Eventually. Eventually supports this by accepting functions that take a single Gomega argument and return zero or more values.

Here’s an example that makes some asssertions and returns a value and error:

    Eventually(func(g Gomega) (Widget, error) {
        ids, err := client.FetchIDs()
        g.Expect(err).NotTo(HaveOccurred())
        g.Expect(ids).To(ContainElement(1138))
        return client.FetchWidget(1138)
    }).Should(Equal(expectedWidget))

will pass only if all the assertions in the polled function pass and the return value satisfied the matcher. Note that the assertions in the body of the polled function must be performed using the passed-in g Gomega object. If you use the global DSL expectations, Eventually will not intercept any failures and the test will fail.

Eventually also supports a special case polling function that takes a single Gomega argument and returns no values. Eventually assumes such a function is making assertions and is designed to work with the Succeed matcher to validate that all assertions have passed.

For example:

    Eventually(func(g Gomega) {
        model, err := client.Find(1138)
        g.Expect(err).NotTo(HaveOccurred())
        g.Expect(model.Reticulate()).To(Succeed())
        g.Expect(model.IsReticulated()).To(BeTrue())
        g.Expect(model.Save()).To(Succeed())
    }).Should(Succeed())

will rerun the function until all assertions pass.

Consistently

Consistently checks that an assertion passes for a period of time. It does this by polling its argument repeatedly during the period. It fails if the matcher ever fails during that period.

For example:

    Consistently(func() []int {
        return thing.MemoryUsage()
    }).Should(BeNumerically("<", 10))

Consistently will poll the passed in function repeatedly and check the return value against the GomegaMatcher. Consistently blocks and only returns when the desired duration has elapsed or if the matcher fails. The default value for the wait-duration is 100 milliseconds. The default polling interval is 10 milliseconds. Like Eventually, you can change these values by passing them in just after your function:

    Consistently(ACTUAL, DURATION, POLLING_INTERVAL).Should(MATCHER)

As with Eventually, these can be time.Durations, string representations of a time.Duration (e.g. "200ms") or float64s that are interpreted as seconds.

Also as with Eventually, Consistently supports chaining WithTimeout and WithPolling in the form of:

    Consistently(ACTUAL).WithTimeout(DURATION).WithPolling(POLLING_INTERVAL).Should(MATCHER)

Consistently tries to capture the notion that something “does not eventually” happen. A common use-case is to assert that no goroutine writes to a channel for a period of time. If you pass Consistently an argument that is not a function, it simply passes that argument to the matcher. So we can assert that:

    Consistently(channel).ShouldNot(Receive())

To assert that nothing gets sent to a channel.

As with Eventually, you can also pass Consistently a function. In fact, Consistently works with the three categories of ACTUAL value outlined for Eventually in the section above.

Developers often try to use runtime.Gosched() to nudge background goroutines to run. This can lead to flaky tests as it is not deterministic that a given goroutine will run during the Gosched. Consistently is particularly handy in these cases: it polls for 100ms which is typically more than enough time for all your Goroutines to run. Yes, this is basically like putting a time.Sleep() in your tests… Sometimes, when making negative assertions in a concurrent world, that’s the best you can do!

Modifying Default Intervals

By default, Eventually will poll every 10 milliseconds for up to 1 second and Consistently will monitor every 10 milliseconds for up to 100 milliseconds. You can modify these defaults across your test suite with:

    SetDefaultEventuallyTimeout(t time.Duration)
    SetDefaultEventuallyPollingInterval(t time.Duration)
    SetDefaultConsistentlyDuration(t time.Duration)
    SetDefaultConsistentlyPollingInterval(t time.Duration)

You can also adjust these global timeouts by setting the GOMEGA_DEFAULT_EVENTUALLY_TIMEOUT, GOMEGA_DEFAULT_EVENTUALLY_POLLING_INTERVAL, GOMEGA_DEFAULT_CONSISTENTLY_DURATION, and GOMEGA_DEFAULT_CONSISTENTLY_POLLING_INTERVAL environment variables to a parseable duration string.

Making Assertions in Helper Functions

While writing custom matchers is an expressive way to make assertions against your code, it is often more convenient to write one-off helper functions like so:

    var _ = Describe("Turbo-encabulator", func() {
        ...
        func assertTurboEncabulatorContains(components ...string) {
            teComponents, err := turboEncabulator.GetComponents()
            Expect(err).NotTo(HaveOccurred())

            Expect(teComponents).To(HaveLen(components))
            for _, component := range components {
                Expect(teComponents).To(ContainElement(component))
            }
        }

        It("should have components", func() {
            assertTurboEncabulatorContains("semi-boloid slots", "grammeters")
        })
    })

This makes your tests more expressive and reduces boilerplate. However, when an assertion in the helper fails the line numbers provided by Gomega are unhelpful. Instead of pointing you to the line in your test that failed, they point you the line in the helper.

To get around this, Gomega provides versions of Expect, Eventually and Consistently named ExpectWithOffset, EventuallyWithOffset and ConsistentlyWithOffset that allow you to specify an offset in the call stack. The offset is the first argument to these functions.

With this, we can rewrite our helper as:

    func assertTurboEncabulatorContains(components ...string) {
        teComponents, err := turboEncabulator.GetComponents()
        ExpectWithOffset(1, err).NotTo(HaveOccurred())

        ExpectWithOffset(1, teComponents).To(HaveLen(components))
        for _, component := range components {
          ExpectWithOffset(1, teComponents).To(ContainElement(component))
        }
    }

Now, failed assertions will point to the correct call to the helper in the test.

Alternatively, you can just use the baseline versions of Expect, Eventually and Consistently and combine them with WithOffset:

    func assertTurboEncabulatorContains(components ...string) {
        teComponents, err := turboEncabulator.GetComponents()
        Expect(err).WithOffset(1).NotTo(HaveOccurred())

        Expect(teComponents).WithOffset(1).To(HaveLen(components))
        for _, component := range components {
          Expect(teComponents).WithOffset(1).To(ContainElement(component))
        }
    }

Provided Matchers

Gomega comes with a bunch of GomegaMatchers. They’re all documented here. If there’s one you’d like to see written either send a pull request or open an issue.

A number of community-supported matchers have appeared as well. A list is maintained on the Gomega wiki.

These docs only go over the positive assertion case (Should), the negative case (ShouldNot) is simply the negation of the positive case. They also use the Ω notation, but - as mentioned above - the Expect notation is equivalent.

Asserting Equivalence

Equal(expected interface{})

    Ω(ACTUAL).Should(Equal(EXPECTED))

uses reflect.DeepEqual to compare ACTUAL with EXPECTED.

reflect.DeepEqual is awesome. It will use == when appropriate (e.g. when comparing primitives) but will recursively dig into maps, slices, arrays, and even your own structs to ensure deep equality. reflect.DeepEqual, however, is strict about comparing types. Both ACTUAL and EXPECTED must have the same type. If you want to compare across different types (e.g. if you’ve defined a type alias) you should use BeEquivalentTo.

It is an error for both ACTUAL and EXPECTED to be nil, you should use BeNil() instead.

When both ACTUAL and EXPECTED are a very long strings, it will attempt to pretty-print the diff and display exactly where they differ.

For asserting equality between numbers of different types, you’ll want to use the BeNumerically() matcher.

BeEquivalentTo(expected interface{})

    Ω(ACTUAL).Should(BeEquivalentTo(EXPECTED))

Like Equal, BeEquivalentTo uses reflect.DeepEqual to compare ACTUAL with EXPECTED. Unlike Equal, however, BeEquivalentTo will first convert ACTUAL’s type to that of EXPECTED before making the comparison with reflect.DeepEqual.

This means that BeEquivalentTo will successfully match equivalent values of different types. This is particularly useful, for example, with type aliases:

    type FoodSrce string

    Ω(FoodSrce("Cheeseboard Pizza")
     ).Should(Equal("Cheeseboard Pizza")) //will fail
    Ω(FoodSrce("Cheeseboard Pizza")
     ).Should(BeEquivalentTo("Cheeseboard Pizza")) //will pass

As with Equal it is an error for both ACTUAL and EXPECTED to be nil, you should use BeNil() instead.

As a rule, you should not use BeEquivalentTo with numbers. Both of the following assertions are true:

    Ω(5.1).Should(BeEquivalentTo(5))
    Ω(5).ShouldNot(BeEquivalentTo(5.1))

the first assertion passes because 5.1 will be cast to an integer and will get rounded down! Such false positives are terrible and should be avoided. Use BeNumerically() to compare numbers instead.

BeIdenticalTo(expected interface{})

    Ω(ACTUAL).Should(BeIdenticalTo(EXPECTED))

Like Equal, BeIdenticalTo compares ACTUAL to EXPECTED for equality. Unlike Equal, however, it uses == to compare values. In practice, this means that primitive values like strings, integers and floats are identical to, as well as pointers to values.

BeIdenticalTo is most useful when you want to assert that two pointers point to the exact same location in memory.

As with Equal it is an error for both ACTUAL and EXPECTED to be nil, you should use BeNil() instead.

BeAssignableToTypeOf(expected interface)

    Ω(ACTUAL).Should(BeAssignableToTypeOf(EXPECTED interface))

succeeds if ACTUAL is a type that can be assigned to a variable with the same type as EXPECTED. It is an error for either ACTUAL or EXPECTED to be nil.

Asserting Presence

BeNil()

    Ω(ACTUAL).Should(BeNil())

succeeds if ACTUAL is, in fact, nil.

BeZero()

    Ω(ACTUAL).Should(BeZero())

succeeds if ACTUAL is the zero value for its type or if ACTUAL is nil.

Asserting Truthiness

BeTrue()

    Ω(ACTUAL).Should(BeTrue())

succeeds if ACTUAL is bool typed and has the value true. It is an error for ACTUAL to not be a bool.

Some matcher libraries have a notion of “truthiness” to assert that an object is present. Gomega is strict, and BeTrue() only works with bools. You can use Ω(ACTUAL).ShouldNot(BeZero()) or Ω(ACTUAL).ShouldNot(BeNil()) to verify object presence.

BeFalse()

    Ω(ACTUAL).Should(BeFalse())

succeeds if ACTUAL is bool typed and has the value false. It is an error for ACTUAL to not be a bool.

Asserting on Errors

HaveOccurred()

    Ω(ACTUAL).Should(HaveOccurred())

succeeds if ACTUAL is a non-nil error. Thus, the typical Go error checking pattern looks like:

    err := SomethingThatMightFail()
    Ω(err).ShouldNot(HaveOccurred())

Succeed()

    Ω(ACTUAL).Should(Succeed())

succeeds if ACTUAL is nil. The intended usage is

    Ω(FUNCTION()).Should(Succeed())

where FUNCTION() is a function call that returns an error-type as its first or only return value. See Handling Errors for a more detailed discussion.

MatchError(expected interface{})

    Ω(ACTUAL).Should(MatchError(EXPECTED))

succeeds if ACTUAL is a non-nil error that matches EXPECTED. EXPECTED must be one of the following:

• A string, in which case ACTUAL.Error() will be compared against EXPECTED.
• A matcher, in which case ACTUAL.Error() is tested against the matcher.
• An error, in which case ACTUAL and EXPECTED are compared via reflect.DeepEqual(). If they are not deeply equal, they are tested by errors.Is(ACTUAL, EXPECTED). (The latter allows to test whether ACTUAL wraps an EXPECTED error.)

Any other type for EXPECTED is an error.

Working with Channels

BeClosed()

    Ω(ACTUAL).Should(BeClosed())

succeeds if ACTUAL is a closed channel. It is an error to pass a non-channel to BeClosed, it is also an error to pass nil.

In order to check whether or not the channel is closed, Gomega must try to read from the channel (even in the ShouldNot(BeClosed()) case). You should keep this in mind if you wish to make subsequent assertions about values coming down the channel.

Also, if you are testing that a buffered channel is closed you must first read all values out of the channel before asserting that it is closed (it is not possible to detect that a buffered-channel has been closed until all its buffered values are read).

Finally, as a corollary: it is an error to check whether or not a send-only channel is closed.

Receive()

    Ω(ACTUAL).Should(Receive(<optionalPointer>))

succeeds if there is a message to be received on actual. Actual must be a channel (and cannot be a send-only channel) – anything else is an error.

Receive returns immediately. It never blocks:

• If there is nothing on the channel c then Ω(c).Should(Receive()) will fail and Ω(c).ShouldNot(Receive()) will pass.
• If there is something on the channel c ready to be read, then Ω(c).Should(Receive()) will pass and Ω(c).ShouldNot(Receive()) will fail.
• If the channel c is closed then Ω(c).Should(Receive()) will fail and Ω(c).ShouldNot(Receive()) will pass.

If you have a go-routine running in the background that will write to channel c, for example:

    go func() {
        time.Sleep(100 * time.Millisecond)
        c <- true
    }()

you can assert that c receives something (anything!) eventually:

    Eventually(c).Should(Receive())

This will timeout if nothing gets sent to c (you can modify the timeout interval as you normally do with Eventually).

A similar use-case is to assert that no go-routine writes to a channel (for a period of time). You can do this with Consistently:

    Consistently(c).ShouldNot(Receive())

Receive also allows you to make assertions on the received object. You do this by passing Receive a matcher:

    Eventually(c).Should(Receive(Equal("foo")))

This assertion will only succeed if c receives an object and that object satisfies Equal("foo"). Note that Eventually will continually poll c until this condition is met. If there are objects coming down the channel that do not satisfy the passed in matcher, they will be pulled off and discarded until an object that does satisfy the matcher is received.

In addition, there are occasions when you need to grab the object sent down the channel (e.g. to make several assertions against the object). To do this, you can ask the Receive matcher for the value passed to the channel by passing it a pointer to a variable of the appropriate type:

    var receivedBagel Bagel
    Eventually(bagelChan).Should(Receive(&receivedBagel))
    Ω(receivedBagel.Contents()).Should(ContainElement("cream cheese"))
    Ω(receivedBagel.Kind()).Should(Equal("sesame"))

Of course, this could have been written as receivedBagel := <-bagelChan - however using Receive makes it easy to avoid hanging the test suite should nothing ever come down the channel. The pointer can point to any variable whose type is assignable from the channel element type, or if the channel type is an interface and the underlying type is assignable to the pointer.

Finally, Receive never blocks. Eventually(c).Should(Receive()) repeatedly polls c in a non-blocking fashion. That means that you cannot use this pattern to verify that a non-blocking send has occurred on the channel - more details at this GitHub issue.

BeSent(value interface{})

    Ω(ACTUAL).Should(BeSent(VALUE))

attempts to send VALUE to the channel ACTUAL without blocking. It succeeds if this is possible.

ACTUAL must be a channel (and cannot be a receive-only channel) that can be sent the type of the VALUE passed into BeSent – anything else is an error. In addition, ACTUAL must not be closed.

BeSent never blocks:

• If the channel c is not ready to receive then Ω(c).Should(BeSent("foo")) will fail immediately.
• If the channel c is eventually ready to receive then Eventually(c).Should(BeSent("foo")) will succeed… presuming the channel becomes ready to receive before Eventually’s timeout.
• If the channel c is closed then Ω(c).Should(BeSent("foo")) and Ω(c).ShouldNot(BeSent("foo")) will both fail immediately.

Of course, VALUE is actually sent to the channel. The point of BeSent is less to make an assertion about the availability of the channel (which is typically an implementation detail that your test should not be concerned with). Rather, the point of BeSent is to make it possible to easily and expressively write tests that can timeout on blocked channel sends.

Working with files

BeAnExistingFile

    Ω(ACTUAL).Should(BeAnExistingFile())

succeeds if a file located at ACTUAL exists.

ACTUAL must be a string representing the filepath.

BeARegularFile

    Ω(ACTUAL).Should(BeARegularFile())

succeeds IFF a file located at ACTUAL exists and is a regular file.

ACTUAL must be a string representing the filepath.

BeADirectory

    Ω(ACTUAL).Should(BeADirectory())

succeeds IFF a file is located at ACTUAL exists and is a directory.

ACTUAL must be a string representing the filepath.

Working with Strings, JSON and YAML

ContainSubstring(substr string, args …interface{})

    Ω(ACTUAL).Should(ContainSubstring(STRING, ARGS...))

succeeds if ACTUAL contains the substring generated by:

    fmt.Sprintf(STRING, ARGS...)

ACTUAL must either be a string, []byte or a Stringer (a type implementing the String() method). Any other input is an error.

Note, of course, that the ARGS... are not required. They are simply a convenience to allow you to build up strings programmatically inline in the matcher.

HavePrefix(prefix string, args …interface{})

    Ω(ACTUAL).Should(HavePrefix(STRING, ARGS...))

succeeds if ACTUAL has the string prefix generated by:

    fmt.Sprintf(STRING, ARGS...)

ACTUAL must either be a string, []byte or a Stringer (a type implementing the String() method). Any other input is an error.

Note, of course, that the ARGS... are not required. They are simply a convenience to allow you to build up strings programmatically inline in the matcher.

HaveSuffix(suffix string, args …interface{})

    Ω(ACTUAL).Should(HaveSuffix(STRING, ARGS...))

succeeds if ACTUAL has the string suffix generated by:

    fmt.Sprintf(STRING, ARGS...)

ACTUAL must either be a string, []byte or a Stringer (a type implementing the String() method). Any other input is an error.

Note, of course, that the ARGS... are not required. They are simply a convenience to allow you to build up strings programmatically inline in the matcher.

MatchRegexp(regexp string, args …interface{})

    Ω(ACTUAL).Should(MatchRegexp(STRING, ARGS...))

succeeds if ACTUAL is matched by the regular expression string generated by:

    fmt.Sprintf(STRING, ARGS...)

ACTUAL must either be a string, []byte or a Stringer (a type implementing the String() method). Any other input is an error. It is also an error for the regular expression to fail to compile.

Note, of course, that the ARGS... are not required. They are simply a convenience to allow you to build up strings programmatically inline in the matcher.

MatchJSON(json interface{})

    Ω(ACTUAL).Should(MatchJSON(EXPECTED))

Both ACTUAL and EXPECTED must be a string, []byte or a Stringer. MatchJSON succeeds if both ACTUAL and EXPECTED are JSON representations of the same object. This is verified by parsing both ACTUAL and EXPECTED and then asserting equality on the resulting objects with reflect.DeepEqual. By doing this MatchJSON avoids any issues related to white space, formatting, and key-ordering.

It is an error for either ACTUAL or EXPECTED to be invalid JSON.

In some cases it is useful to match two JSON strings while ignoring list order. For this you can use the community maintained MatchUnorderedJSON matcher.

MatchXML(xml interface{})

    Ω(ACTUAL).Should(MatchXML(EXPECTED))

Both ACTUAL and EXPECTED must be a string, []byte or a Stringer. MatchXML succeeds if both ACTUAL and EXPECTED are XML representations of the same object. This is verified by parsing both ACTUAL and EXPECTED and then asserting equality on the resulting objects with reflect.DeepEqual. By doing this MatchXML avoids any issues related to white space or formatting.

It is an error for either ACTUAL or EXPECTED to be invalid XML.

MatchYAML(yaml interface{})

    Ω(ACTUAL).Should(MatchYAML(EXPECTED))

Both ACTUAL and EXPECTED must be a string, []byte or a Stringer. MatchYAML succeeds if both ACTUAL and EXPECTED are YAML representations of the same object. This is verified by parsing both ACTUAL and EXPECTED and then asserting equality on the resulting objects with reflect.DeepEqual. By doing this MatchYAML avoids any issues related to white space, formatting, and key-ordering.

It is an error for either ACTUAL or EXPECTED to be invalid YAML.

Working with Collections

BeEmpty()

    Ω(ACTUAL).Should(BeEmpty())

succeeds if ACTUAL is, in fact, empty. ACTUAL must be of type string, array, map, chan, or slice. It is an error for it to have any other type.

HaveLen(count int)

    Ω(ACTUAL).Should(HaveLen(INT))

succeeds if the length of ACTUAL is INT. ACTUAL must be of type string, array, map, chan, or slice. It is an error for it to have any other type.

HaveCap(count int)

    Ω(ACTUAL).Should(HaveCap(INT))

succeeds if the capacity of ACTUAL is INT. ACTUAL must be of type array, chan, or slice. It is an error for it to have any other type.

ContainElement(element interface{})

    Ω(ACTUAL).Should(ContainElement(ELEMENT))

succeeds if ACTUAL contains an element that equals ELEMENT. ACTUAL must be an array, slice, or map – anything else is an error. For maps ContainElement searches through the map’s values (not keys!).

By default ContainElement() uses the Equal() matcher under the hood to assert equality between ACTUAL’s elements and ELEMENT. You can change this, however, by passing ContainElement a GomegaMatcher. For example, to check that a slice of strings has an element that matches a substring:

    Ω([]string{"Foo", "FooBar"}
     ).Should(ContainElement(ContainSubstring("Bar")))

ContainElements(element …interface{})

    Ω(ACTUAL).Should(ContainElements(ELEMENT1, ELEMENT2, ELEMENT3, ...))

or

    Ω(ACTUAL).Should(
        ContainElements([]SOME_TYPE{ELEMENT1, ELEMENT2, ELEMENT3, ...}))

succeeds if ACTUAL contains the elements passed into the matcher. The ordering of the elements does not matter.

By default ContainElements() uses Equal() to match the elements, however custom matchers can be passed in instead. Here are some examples:

    Ω([]string{"Foo", "FooBar"}).Should(ContainElements("FooBar"))
    Ω([]string{"Foo", "FooBar"}).Should(ContainElements(ContainSubstring("Bar"), "Foo"))

Actual must be an array, slice or map. For maps, ContainElements matches against the map’s values.

You typically pass variadic arguments to ContainElements (as in the examples above). However, if you need to pass in a slice you can provided that it is the only element passed in to ContainElements:

    Ω([]string{"Foo", "FooBar"}).Should(ContainElements([]string{"FooBar", "Foo"}))

Note that Go’s type system does not allow you to write this as ContainElements([]string{"FooBar", "Foo"}...) as []string and []interface{} are different types - hence the need for this special rule.

The difference between the ContainElements and ConsistOf matchers is that the latter is more restrictive because the ConsistOf matcher checks additionally that the ACTUAL elements and the elements passed into the matcher have the same length.

BeElementOf(elements …interface{})

    Ω(ACTUAL).Should(BeElementOf(ELEMENT1, ELEMENT2, ELEMENT3, ...))

succeeds if ACTUAL equals one of the elements passed into the matcher. When a single element ELEMENT of type array or slice is passed into the matcher, BeElementOf succeeds if ELEMENT contains an element that equals ACTUAL (reverse of ContainElement). BeElementOf always uses the Equal() matcher under the hood to assert equality.

ConsistOf(element …interface{})

    Ω(ACTUAL).Should(ConsistOf(ELEMENT1, ELEMENT2, ELEMENT3, ...))

or

    Ω(ACTUAL).Should(
        ConsistOf([]SOME_TYPE{ELEMENT1, ELEMENT2, ELEMENT3, ...}))

succeeds if ACTUAL contains precisely the elements passed into the matcher. The ordering of the elements does not matter.

By default ConsistOf() uses Equal() to match the elements, however custom matchers can be passed in instead. Here are some examples:

    Ω([]string{"Foo", "FooBar"}).Should(ConsistOf("FooBar", "Foo"))
    Ω([]string{"Foo", "FooBar"}).Should(ConsistOf(ContainSubstring("Bar"), "Foo"))
    Ω([]string{"Foo", "FooBar"}).Should(ConsistOf(ContainSubstring("Foo"), ContainSubstring("Foo")))

Actual must be an array, slice or map. For maps, ConsistOf matches against the map’s values.

You typically pass variadic arguments to ConsistOf (as in the examples above). However, if you need to pass in a slice you can provided that it is the only element passed in to ConsistOf:

    Ω([]string{"Foo", "FooBar"}).Should(ConsistOf([]string{"FooBar", "Foo"}))

Note that Go’s type system does not allow you to write this as ConsistOf([]string{"FooBar", "Foo"}...) as []string and []interface{} are different types - hence the need for this special rule.

HaveKey(key interface{})

    Ω(ACTUAL).Should(HaveKey(KEY))

succeeds if ACTUAL is a map with a key that equals KEY. It is an error for ACTUAL to not be a map.

By default HaveKey() uses the Equal() matcher under the hood to assert equality between ACTUAL’s keys and KEY. You can change this, however, by passing HaveKey a GomegaMatcher. For example, to check that a map has a key that matches a regular expression:

    Ω(map[string]string{"Foo": "Bar", "BazFoo": "Duck"}).Should(HaveKey(MatchRegexp(`.+Foo$`)))

HaveKeyWithValue(key interface{}, value interface{})

    Ω(ACTUAL).Should(HaveKeyWithValue(KEY, VALUE))

succeeds if ACTUAL is a map with a key that equals KEY mapping to a value that equals VALUE. It is an error for ACTUAL to not be a map.

By default HaveKeyWithValue() uses the Equal() matcher under the hood to assert equality between ACTUAL’s keys and KEY and between the associated value and VALUE. You can change this, however, by passing HaveKeyWithValue a GomegaMatcher for either parameter. For example, to check that a map has a key that matches a regular expression and which is also associated with a value that passes some numerical threshold:

    Ω(map[string]int{"Foo": 3, "BazFoo": 4}).Should(HaveKeyWithValue(MatchRegexp(`.+Foo$`), BeNumerically(">", 3)))

HaveField(field interface{}, value interface{})

    Ω(ACTUAL).Should(HaveField(FIELD, VALUE))

succeeds if ACTUAL is a struct with a value that can be traversed via FIELD that equals VALUE. It is an error for ACTUAL to not be a struct.

By default HaveField() uses the Equal() matcher under the hood to assert equality between the extracted value and VALUE. You can change this, however, by passing HaveField a GomegaMatcher for VALUE.

FIELD allows you to access fields within the ACTUAL struct. Nested structs can be accessed using the . delimiter. HaveField() also allows you to invoke methods on the struct by adding a () suffix to the FIELD - these methods must take no arguments and return exactly one value. For example consider the following types:

type Book struct {
    Title string
    Author Person
}

type Person struct {
    Name string
    DOB time.Time
}

and an instance book var book = Book{...} - you can use HaveField to make assertions like:

    Ω(book).Should(HaveField("Title", "Les Miserables"))
    Ω(book).Should(HaveField("Title", ContainSubstring("Les Mis")))
    Ω(book).Should(HaveField("Author.Name", "Victor Hugo"))
    Ω(book).Should(HaveField("Author.DOB.Year()", BeNumerically("<", 1900)))

HaveField can pair powerfully with a collection matcher like ContainElement. To assert that a list of books as at least one element with an author born in February you could write:

    Ω(books).Should(ContainElement(HaveField("Author.DOB.Month()", Equal(2))))

If you want to make lots of complex assertions against the fields of a struct take a look at the gstructpackage package documented below.

Working with Numbers and Times

BeNumerically(comparator string, compareTo …interface{})

    Ω(ACTUAL).Should(BeNumerically(COMPARATOR_STRING, EXPECTED, <THRESHOLD>))

performs numerical assertions in a type-agnostic way. ACTUAL and EXPECTED should be numbers, though the specific type of number is irrelevant (float32, float64, uint8, etc…). It is an error for ACTUAL or EXPECTED to not be a number.

There are six supported comparators:

• Ω(ACTUAL).Should(BeNumerically("==", EXPECTED)): asserts that ACTUAL and EXPECTED are numerically equal.

• Ω(ACTUAL).Should(BeNumerically("~", EXPECTED, <THRESHOLD>)): asserts that ACTUAL and EXPECTED are within <THRESHOLD> of one another. By default <THRESHOLD> is 1e-8 but you can specify a custom value.

• Ω(ACTUAL).Should(BeNumerically(">", EXPECTED)): asserts that ACTUAL is greater than EXPECTED.

• Ω(ACTUAL).Should(BeNumerically(">=", EXPECTED)): asserts that ACTUAL is greater than or equal to EXPECTED.

• Ω(ACTUAL).Should(BeNumerically("<", EXPECTED)): asserts that ACTUAL is less than EXPECTED.

• Ω(ACTUAL).Should(BeNumerically("<=", EXPECTED)): asserts that ACTUAL is less than or equal to EXPECTED.

Any other comparator is an error.

BeTemporally(comparator string, compareTo time.Time, threshold …time.Duration)

    Ω(ACTUAL).Should(BeTemporally(COMPARATOR_STRING, EXPECTED_TIME, <THRESHOLD_DURATION>))

performs time-related assertions. ACTUAL must be a time.Time.

There are six supported comparators:

• Ω(ACTUAL).Should(BeTemporally("==", EXPECTED_TIME)): asserts that ACTUAL and EXPECTED_TIME are identical time.Times.

• Ω(ACTUAL).Should(BeTemporally("~", EXPECTED_TIME, <THRESHOLD_DURATION>)): asserts that ACTUAL and EXPECTED_TIME are within <THRESHOLD_DURATION> of one another. By default <THRESHOLD_DURATION> is time.Millisecond but you can specify a custom value.

• Ω(ACTUAL).Should(BeTemporally(">", EXPECTED_TIME)): asserts that ACTUAL is after EXPECTED_TIME.

• Ω(ACTUAL).Should(BeTemporally(">=", EXPECTED_TIME)): asserts that ACTUAL is after or at EXPECTED_TIME.

• Ω(ACTUAL).Should(BeTemporally("<", EXPECTED_TIME)): asserts that ACTUAL is before EXPECTED_TIME.

• Ω(ACTUAL).Should(BeTemporally("<=", EXPECTED_TIME)): asserts that ACTUAL is before or at EXPECTED_TIME.

Any other comparator is an error.

Working with HTTP responses

HaveHTTPStatus(expected interface{})

    Expect(ACTUAL).To(HaveHTTPStatus(EXPECTED, ...))

succeeds if the Status or StatusCode field of an HTTP response matches.

ACTUAL must be either a *http.Response or *httptest.ResponseRecorder.

EXPECTED must be one or more int or string types. An int is compared to StatusCode and a string is compared to Status. The matcher succeeds if any of the EXPECTED values match.

Here are some examples:

• Expect(resp).To(HaveHTTPStatus(http.StatusOK, http.StatusNoContext)): asserts that resp.StatusCode == 200 or resp.StatusCode == 204

• Expect(resp).To(HaveHTTPStatus("404 Not Found")): asserts that resp.Status == "404 Not Found".

HaveHTTPBody(expected interface{})

    Expect(ACTUAL).To(HaveHTTPBody(EXPECTED))

Succeeds if the body of an HTTP Response matches.

ACTUAL must be either a *http.Response or *httptest.ResponseRecorder.

EXPECTED must be one of the following:

• A string
• A []byte
• A matcher, in which case the matcher will be called with the body as a []byte.

Here are some examples:

• Expect(resp).To(HaveHTTPBody("bar")): asserts that when resp.Body is read, it will equal bar.

• Expect(resp).To(HaveHTTPBody(MatchJSON("{\"some\":\"json\"")): asserts that when resp.Body is read, the MatchJSON matches it to {"some":"json"}.

Note that the body is an io.ReadCloser and the HaveHTTPBody() will read it and the close it. This means that subsequent attempts to read the body may have unexpected results.

HaveHTTPHeaderWithValue(key string, value interface{})

    Expect(ACTUAL).To(HaveHTTPHeaderWithValue(KEY, VALUE))

Succeeds if the HTTP Response has a matching header and value.

ACTUAL must be either a *http.Response or *httptest.ResponseRecorder.

KEY must be a string. It is passed to http.Header.Get(key string), and will have the same behaviors regarding order of headers and capitalization.

VALUE must be one of the following:

• A string
• A matcher, in which case the matcher will be called to match the value.

Here are some examples:

• Expect(resp).To(HaveHTTPHeaderWithValue("Content-Type", "application/json")): asserts that the Content-Type header has exactly the value application/json.

• Expect(resp).To(HaveHTTPHeaderWithValue(ContainsSubstring("json"))): asserts that the Content-Type header contains the substring json.

Asserting on Panics

Panic()

    Ω(ACTUAL).Should(Panic())

succeeds if ACTUAL is a function that, when invoked, panics. ACTUAL must be a function that takes no arguments and returns no result – any other type for ACTUAL is an error.

PanicWith()

    Ω(ACTUAL).Should(PanicWith(VALUE))

succeeds if ACTUAL is a function that, when invoked, panics with a value of VALUE. ACTUAL must be a function that takes no arguments and returns no result – any other type for ACTUAL is an error.

By default PanicWith() uses the Equal() matcher under the hood to assert equality between ACTUAL’s panic value and VALUE. You can change this, however, by passing PanicWith a GomegaMatcher. For example, to check that the panic value matches a regular expression:

    Ω(func() { panic("FooBarBaz") }).Should(PanicWith(MatchRegexp(`.+Baz$`)))

Composing Matchers

You may form larger matcher expressions using the following operators: And(), Or(), Not() and WithTransform().

Note: And() and Or() can also be referred to as SatisfyAll() and SatisfyAny(), respectively.

With these operators you can express multiple requirements in a single Expect() or Eventually() statement. For example:

    Expect(number).To(SatisfyAll(
                BeNumerically(">", 0),
                BeNumerically("<", 10)))

    Expect(msg).To(SatisfyAny(
                Equal("Success"),
                MatchRegexp(`^Error .+$`)))

It can also provide a lightweight syntax to create new matcher types from existing ones. For example:

    func BeBetween(min, max int) GomegaMatcher {
        return SatisfyAll(
                BeNumerically(">", min),
                BeNumerically("<", max))
    }

    Ω(number).Should(BeBetween(0, 10))

And(matchers …GomegaMatcher)

SatisfyAll(matchers …GomegaMatcher)

    Ω(ACTUAL).Should(And(MATCHER1, MATCHER2, ...))

or

    Ω(ACTUAL).Should(SatisfyAll(MATCHER1, MATCHER2, ...))

succeeds if ACTUAL satisfies all of the specified matchers (similar to a logical AND).

Tests the given matchers in order, returning immediately if one fails, without needing to test the remaining matchers.

Or(matchers …GomegaMatcher)

SatisfyAny(matchers …GomegaMatcher)

    Ω(ACTUAL).Should(Or(MATCHER1, MATCHER2, ...))

or

    Ω(ACTUAL).Should(SatisfyAny(MATCHER1, MATCHER2, ...))

succeeds if ACTUAL satisfies any of the specified matchers (similar to a logical OR).

Tests the given matchers in order, returning immediately if one succeeds, without needing to test the remaining matchers.

Not(matcher GomegaMatcher)

    Ω(ACTUAL).Should(Not(MATCHER))

succeeds if ACTUAL does not satisfy the specified matcher (similar to a logical NOT).

WithTransform(transform interface{}, matcher GomegaMatcher)

    Ω(ACTUAL).Should(WithTransform(TRANSFORM, MATCHER))

succeeds if applying the TRANSFORM function to ACTUAL (i.e. the value of TRANSFORM(ACTUAL)) will satisfy the given MATCHER. For example:

    GetColor := func(e Element) Color { return e.Color }

    Ω(element).Should(WithTransform(GetColor, Equal(BLUE)))

Or the same thing expressed by introducing a new, lightweight matcher:

    // HaveColor returns a matcher that expects the element to have the given color.
    func HaveColor(c Color) GomegaMatcher {
        return WithTransform(func(e Element) Color {
            return e.Color
        }, Equal(c))
    }

    Ω(element).Should(HaveColor(BLUE)))

TRANSFORM functions optionally can return an additional error value in case a transformation is not possible, avoiding the need to panic. Returning errors can be useful when using WithTransform to build lightweight matchers that accept different value types and that can gracefully fail when presented the wrong value type.

As before, such a TRANSFORM expects a single actual value. But now it returns the transformed value together with an error value. This follows the common Go idiom to communicate errors via an explicit, separate return value.

The following lightweight matcher expects to be used either on a Sprocket value or *Sprocket pointer. It gracefully fails when the actual value is something else.

    // HaveSprocketName returns a matcher that expects the actual value to be
    // either a Sprocket or a *Sprocket, having the specified name.
    func HaveSprocketName(name string) GomegaMatcher {
        return WithTransform(
            func(actual interface{}) (string, error) {
                switch sprocket := actual.(type) {
                case *Sprocket:
                    return Sprocket.Name, nil
                case Sprocket:
                    return Sprocket.Name, nil
                default:
                    return "", fmt.Errorf("HaveSprocketName expects a Sprocket or *Sprocket, but got %T", actual)
                }
            }, Equal(name))
    }

    Ω(element).Should(HaveSprocketName("gomega")))

Satisfy(predicate interface{})

    Ω(ACTUAL).Should(Satisfy(PREDICATE))

succeeds if applying the PREDICATE function to ACTUAL (i.e. the value of PREDICATE(ACTUAL)) will return true. For example:

    IsEven := func(i int) bool { return i%2==0 }

    Ω(number).Should(Satisfy(IsEven))

Adding Your Own Matchers

A matcher, in Gomega, is any type that satisfies the GomegaMatcher interface:

    type GomegaMatcher interface {
        Match(actual interface{}) (success bool, err error)
        FailureMessage(actual interface{}) (message string)
        NegatedFailureMessage(actual interface{}) (message string)
    }

For the simplest cases, new matchers can be created by composition.

But writing domain-specific custom matchers is also trivial and highly encouraged. Let’s work through an example.

The GomegaMatcher interface is defined in the types subpackage.

A Custom Matcher: RepresentJSONifiedObject(EXPECTED interface{})

Say you’re working on a JSON API and you want to assert that your server returns the correct JSON representation. Rather than marshal/unmarshal JSON in your tests, you want to write an expressive matcher that checks that the received response is a JSON representation for the object in question. This is what the RepresentJSONifiedObject matcher could look like:

    package json_response_matcher

    import (
        "github.com/onsi/gomega/types"

        "encoding/json"
        "fmt"
        "net/http"
        "reflect"
    )

    func RepresentJSONifiedObject(expected interface{}) types.GomegaMatcher {
        return &representJSONMatcher{
            expected: expected,
        }
    }

    type representJSONMatcher struct {
        expected interface{}
    }

    func (matcher *representJSONMatcher) Match(actual interface{}) (success bool, err error) {
        response, ok := actual.(*http.Response)
        if !ok {
            return false, fmt.Errorf("RepresentJSONifiedObject matcher expects an http.Response")
        }

        pointerToObjectOfExpectedType := reflect.New(reflect.TypeOf(matcher.expected)).Interface()
        err = json.NewDecoder(response.Body).Decode(pointerToObjectOfExpectedType)

        if err != nil {
            return false, fmt.Errorf("Failed to decode JSON: %s", err.Error())
        }

        decodedObject := reflect.ValueOf(pointerToObjectOfExpectedType).Elem().Interface()

        return reflect.DeepEqual(decodedObject, matcher.expected), nil
    }

    func (matcher *representJSONMatcher) FailureMessage(actual interface{}) (message string) {
        return fmt.Sprintf("Expected\n\t%#v\nto contain the JSON representation of\n\t%#v", actual, matcher.expected)
    }

    func (matcher *representJSONMatcher) NegatedFailureMessage(actual interface{}) (message string) {
        return fmt.Sprintf("Expected\n\t%#v\nnot to contain the JSON representation of\n\t%#v", actual, matcher.expected)
    }

Let’s break this down:

• Most matchers have a constructor function that returns an instance of the matcher. In this case we’ve created RepresentJSONifiedObject. Where possible, your constructor function should take explicit types or interfaces. For our use case, however, we need to accept any possible expected type so RepresentJSONifiedObject takes an argument with the generic interface{} type.
• The constructor function then initializes and returns an instance of our matcher: the representJSONMatcher. These rarely need to be exported outside of your matcher package.
• The representJSONMatcher must satisfy the GomegaMatcher interface. It does this by implementing the Match, FailureMessage, and NegatedFailureMessage method:
  • If the GomegaMatcher receives invalid inputs Match returns a non-nil error explaining the problems with the input. This allows Gomega to fail the assertion whether the assertion is for the positive or negative case.
  • If the actual and expected values match, Match should return true.
  • Similarly, if the actual and expected values do not match, Match should return false.
  • If the GomegaMatcher was testing the Should case, and Match returned false, FailureMessage will be called to print a message explaining the failure.
  • Likewise, if the GomegaMatcher was testing the ShouldNot case, and Match returned true, NegatedFailureMessage will be called.
  • It is guaranteed that FailureMessage and NegatedFailureMessage will only be called after Match, so you can save off any state you need to compute the messages in Match.
• Finally, it is common for matchers to make extensive use of the reflect library to interpret the generic inputs they receive. In this case, the representJSONMatcher goes through some reflect gymnastics to create a pointer to a new object with the same type as the expected object, read and decode JSON from actual into that pointer, and then deference the pointer and compare the result to the expected object.

You might test drive this matcher while writing it using Ginkgo. Your test might look like:

    package json_response_matcher_test

    import (
        . "github.com/onsi/ginkgo"
        . "github.com/onsi/gomega"
        . "jsonresponsematcher"

        "bytes"
        "encoding/json"
        "io/ioutil"
        "net/http"
        "strings"

        "testing"
    )

    func TestCustomMatcher(t *testing.T) {
        RegisterFailHandler(Fail)
        RunSpecs(t, "Custom Matcher Suite")
    }

    type Book struct {
        Title  string `json:"title"`
        Author string `json:"author"`
    }

    var _ = Describe("RepresentJSONified Object", func() {
        var (
            book     Book
            bookJSON []byte
            response *http.Response
        )

        BeforeEach(func() {
            book = Book{
                Title:  "Les Miserables",
                Author: "Victor Hugo",
            }

            var err error
            bookJSON, err = json.Marshal(book)
            Ω(err).ShouldNot(HaveOccurred())
        })

        Context("when actual is not an http response", func() {
            It("should error", func() {
                _, err := RepresentJSONifiedObject(book).Match("not a response")
                Ω(err).Should(HaveOccurred())
            })
        })

        Context("when actual is an http response", func() {
            BeforeEach(func() {
                response = &http.Response{}
            })

            Context("with a body containing the JSON representation of actual", func() {
                BeforeEach(func() {
                    response.ContentLength = int64(len(bookJSON))
                    response.Body = ioutil.NopCloser(bytes.NewBuffer(bookJSON))
                })

                It("should succeed", func() {
                    Ω(response).Should(RepresentJSONifiedObject(book))
                })
            })

            Context("with a body containing the JSON representation of something else", func() {
                BeforeEach(func() {
                    reader := strings.NewReader(`{}`)
                    response.ContentLength = int64(reader.Len())
                    response.Body = ioutil.NopCloser(reader)
                })

                It("should fail", func() {
                    Ω(response).ShouldNot(RepresentJSONifiedObject(book))
                })
            })

            Context("with a body containing invalid JSON", func() {
                BeforeEach(func() {
                    reader := strings.NewReader(`floop`)
                    response.ContentLength = int64(reader.Len())
                    response.Body = ioutil.NopCloser(reader)
                })

                It("should error", func() {
                    _, err := RepresentJSONifiedObject(book).Match(response)
                    Ω(err).Should(HaveOccurred())
                })
            })
        })
    })

This also offers an example of what using the matcher would look like in your tests. Note that testing the cases when the matcher returns an error involves creating the matcher and invoking Match manually (instead of using an Ω or Expect assertion).

Aborting Eventually/Consistently

There are sometimes instances where Eventually or Consistently should stop polling a matcher because the result of the match simply cannot change.

For example, consider a test that looks like:

    Eventually(myChannel).Should(Receive(Equal("bar")))

Eventually will repeatedly invoke the Receive matcher against myChannel until the match succeeds. However, if the channel becomes closed there is no way for the match to ever succeed. Allowing Eventually to continue polling is inefficient and slows the test suite down.

To get around this, a matcher can optionally implement:

    MatchMayChangeInTheFuture(actual interface{}) bool

This is not part of the GomegaMatcher interface and, in general, most matchers do not need to implement MatchMayChangeInTheFuture.

If implemented, however, MatchMayChangeInTheFuture will be called with the appropriate actual value by Eventually and Consistently after the call to Match during every polling interval. If MatchMayChangeInTheFuture returns true, Eventually and Consistently will continue polling. If, however, MatchMayChangeInTheFuture returns false, Eventually and Consistently will stop polling and either fail or pass as appropriate.

If you’d like to look at a simple example of MatchMayChangeInTheFuture check out gexec’s Exit matcher. Here, MatchMayChangeInTheFuture returns true if the gexec.Session under test has not exited yet, but returns false if it has. Because of this, if a process exits with status code 3, but an assertion is made of the form:

    Eventually(session, 30).Should(gexec.Exit(0))

Eventually will not block for 30 seconds but will return (and fail, correctly) as soon as the mismatched exit code arrives!

Note: Eventually and Consistently only exercise the MatchMayChangeInTheFuture method if they are passed a bare value. If they are passed functions to be polled it is not possible to guarantee that the return value of the function will not change between polling intervals. In this case, MatchMayChangeInTheFuture is not called and the polling continues until either a match is found or the timeout elapses.

Contributing to Gomega

Contributions are more than welcome. Either open an issue for a matcher you’d like to see or, better yet, test drive the matcher and send a pull request.

When adding a new matcher please mimic the style use in Gomega’s current matchers: you should use the format package to format your output, put the matcher and its tests in the matchers package, and the constructor in the matchers.go file in the top-level package.

ghttp: Testing HTTP Clients

The ghttp package provides support for testing http clients. The typical pattern in Go for testing http clients entails spinning up an httptest.Server using the net/http/httptest package and attaching test-specific handlers that perform assertions.

ghttp provides ghttp.Server - a wrapper around httptest.Server that allows you to easily build up a stack of test handlers. These handlers make assertions against the incoming request and return a pre-fabricated response. ghttp provides a number of prebuilt handlers that cover the most common assertions. You can combine these handlers to build out full-fledged assertions that test multiple aspects of the incoming requests.

The goal of this documentation is to provide you with an adequate mental model to use ghttp correctly. For a full reference of all the available handlers and the various methods on ghttp.Server look at the godoc documentation.

Making assertions against an incoming request

Let’s start with a simple example. Say you are building an API client that provides a FetchSprockets(category string) method that makes an http request to a remote server to fetch sprockets of a given category.

For now, let’s not worry about the values returned by FetchSprockets but simply assert that the correct request was made. Here’s the setup for our ghttp-based Ginkgo test:

    Describe("The sprockets client", func() {
        var server *ghttp.Server
        var client *sprockets.Client

        BeforeEach(func() {
            server = ghttp.NewServer()
            client = sprockets.NewClient(server.URL())
        })

        AfterEach(func() {
            //shut down the server between tests
            server.Close()
        })
    })

Note that the server’s URL is auto-generated and varies between test runs. Because of this, you must always inject the server URL into your client. Let’s add a simple test that asserts that FetchSprockets hits the correct endpoint with the correct HTTP verb:

    Describe("The sprockets client", func() {
        //...see above

        Describe("fetching sprockets", func() {
            BeforeEach(func() {
                server.AppendHandlers(
                    ghttp.VerifyRequest("GET", "/sprockets"),
                )
            })

            It("should make a request to fetch sprockets", func() {
                client.FetchSprockets("")
                Ω(server.ReceivedRequests()).Should(HaveLen(1))
            })
        })
    })

Here we append a VerifyRequest handler to the server and call client.FetchSprockets. This call (assuming it’s a blocking call) will make a round-trip to the test server before returning. The test server receives the request and passes it through the VerifyRequest handler which will validate that the request is a GET request hitting the /sprockets endpoint. If it’s not, the test will fail.

Note that the test can pass trivially if client.FetchSprockets() doesn’t actually make a request. To guard against this you can assert that the server has actually received a request. All the requests received by the server are saved off and made available via server.ReceivedRequests(). We use this to assert that there should have been exactly one received requests.

Guarding against the trivial “false positive” case outlined above isn’t really necessary. Just good practice when test driving.

Let’s add some more to our example. Let’s make an assertion that FetchSprockets can request sprockets filtered by a particular category:

    Describe("The sprockets client", func() {
        //...see above

        Describe("fetching sprockets", func() {
            BeforeEach(func() {
                server.AppendHandlers(
                    ghttp.VerifyRequest("GET", "/sprockets", "category=encabulators"),
                )
            })

            It("should make a request to fetch sprockets", func() {
                client.FetchSprockets("encabulators")
                Ω(server.ReceivedRequests()).Should(HaveLen(1))
            })
        })
    })

ghttp.VerifyRequest takes an optional third parameter that is matched against the request URL’s RawQuery.

Let’s extend the example some more. In addition to asserting that the request is a GET request to the correct endpoint with the correct query params, let’s also assert that it includes the correct BasicAuth information and a correct custom header. Here’s the complete example:

    Describe("The sprockets client", func() {
        var (
            server *ghttp.Server
            client *sprockets.Client
            username, password string
        )

        BeforeEach(func() {
            username, password = "gopher", "tacoshell"
            server = ghttp.NewServer()
            client = sprockets.NewClient(server.URL(), username, password)
        })

        AfterEach(func() {
            server.Close()
        })

        Describe("fetching sprockets", func() {
            BeforeEach(func() {
                server.AppendHandlers(
                    ghttp.CombineHandlers(
                        ghttp.VerifyRequest("GET", "/sprockets", "category=encabulators"),
                        ghttp.VerifyBasicAuth(username, password),
                        ghttp.VerifyHeader(http.Header{
                            "X-Sprocket-API-Version": []string{"1.0"},
                        }),
                    )
                )
            })

            It("should make a request to fetch sprockets", func() {
                client.FetchSprockets("encabulators")
                Ω(server.ReceivedRequests()).Should(HaveLen(1))
            })
        })
    })

This example combines multiple ghttp verify handlers using ghttp.CombineHandlers. Under the hood, this returns a new handler that wraps and invokes the three passed in verify handlers. The request sent by the client will pass through each of these verify handlers and must pass them all for the test to pass.

Note that you can easily add your own verify handler into the mix. Just pass in a regular http.HandlerFunc and make assertions against the received request.

It’s important to understand that you must pass AppendHandlers one handler per incoming request (see below). In order to apply multiple handlers to a single request we must first combine them with ghttp.CombineHandlers and then pass that one wrapper handler in to AppendHandlers.

Providing responses

So far, we’ve only made assertions about the outgoing request. Clients are also responsible for parsing responses and returning valid data. Let’s say that FetchSprockets() returns two things: a slice []Sprocket and an error. Here’s what a happy path test that asserts the correct data is returned might look like:

    Describe("The sprockets client", func() {
        //...
        Describe("fetching sprockets", func() {
            BeforeEach(func() {
                server.AppendHandlers(
                    ghttp.CombineHandlers(
                        ghttp.VerifyRequest("GET", "/sprockets", "category=encabulators"),
                        ghttp.VerifyBasicAuth(username, password),
                        ghttp.VerifyHeader(http.Header{
                            "X-Sprocket-API-Version": []string{"1.0"},
                        }),
                        ghttp.RespondWith(http.StatusOK, `[
                            {"name": "entropic decoupler", "color": "red"},
                            {"name": "defragmenting ramjet", "color": "yellow"}
                        ]`),
                    )
                )
            })

            It("should make a request to fetch sprockets", func() {
                sprockets, err := client.FetchSprockets("encabulators")
                Ω(err).ShouldNot(HaveOccurred())
                Ω(sprockets).Should(Equal([]Sprocket{
                    sprockets.Sprocket{Name: "entropic decoupler", Color: "red"},
                    sprockets.Sprocket{Name: "defragmenting ramjet", Color: "yellow"},
                }))
            })
        })
    })

We use ghttp.RespondWith to specify the response return by the server. In this case we’re passing back a status code of 200 (http.StatusOK) and a pile of JSON. We then assert, in the test, that the client succeeds and returns the correct set of sprockets.

The fact that details of the JSON encoding are bleeding into this test is somewhat unfortunate, and there’s a lot of repetition going on. ghttp provides a RespondWithJSONEncoded handler that accepts an arbitrary object and JSON encodes it for you. Here’s a cleaner test:

    Describe("The sprockets client", func() {
        //...
        Describe("fetching sprockets", func() {
            var returnedSprockets []Sprocket
            BeforeEach(func() {
                returnedSprockets = []Sprocket{
                    sprockets.Sprocket{Name: "entropic decoupler", Color: "red"},
                    sprockets.Sprocket{Name: "defragmenting ramjet", Color: "yellow"},
                }

                server.AppendHandlers(
                    ghttp.CombineHandlers(
                        ghttp.VerifyRequest("GET", "/sprockets", "category=encabulators"),
                        ghttp.VerifyBasicAuth(username, password),
                        ghttp.VerifyHeader(http.Header{
                            "X-Sprocket-API-Version": []string{"1.0"},
                        }),
                        ghttp.RespondWithJSONEncoded(http.StatusOK, returnedSprockets),
                    )
                )
            })

            It("should make a request to fetch sprockets", func() {
                sprockets, err := client.FetchSprockets("encabulators")
                Ω(err).ShouldNot(HaveOccurred())
                Ω(sprockets).Should(Equal(returnedSprockets))
            })
        })
    })

Testing different response scenarios

Our test currently only handles the happy path where the server returns a 200. We should also test a handful of sad paths. In particular, we’d like to return a SprocketsErrorNotFound error when the server 404s and a SprocketsErrorUnauthorized error when the server returns a 401. But how to do this without redefining our server handler three times?

ghttp provides RespondWithPtr and RespondWithJSONEncodedPtr for just this use case. Both take pointers to status codes and respond bodies (objects for the JSON case). Here’s the more complete test:

    Describe("The sprockets client", func() {
        //...
        Describe("fetching sprockets", func() {
            var returnedSprockets []Sprocket
            var statusCode int

            BeforeEach(func() {
                returnedSprockets = []Sprocket{
                    sprockets.Sprocket{Name: "entropic decoupler", Color: "red"},
                    sprockets.Sprocket{Name: "defragmenting ramjet", Color: "yellow"},
                }

                server.AppendHandlers(
                    ghttp.CombineHandlers(
                        ghttp.VerifyRequest("GET", "/sprockets", "category=encabulators"),
                        ghttp.VerifyBasicAuth(username, password),
                        ghttp.VerifyHeader(http.Header{
                            "X-Sprocket-API-Version": []string{"1.0"},
                        }),
                        ghttp.RespondWithJSONEncodedPtr(&statusCode, &returnedSprockets),
                    )
                )
            })

            Context("when the request succeeds", func() {
                BeforeEach(func() {
                    statusCode = http.StatusOK
                })

                It("should return the fetched sprockets without erroring", func() {
                    sprockets, err := client.FetchSprockets("encabulators")
                    Ω(err).ShouldNot(HaveOccurred())
                    Ω(sprockets).Should(Equal(returnedSprockets))
                })
            })

            Context("when the response is unauthorized", func() {
                BeforeEach(func() {
                    statusCode = http.StatusUnauthorized
                })

                It("should return the SprocketsErrorUnauthorized error", func() {
                    sprockets, err := client.FetchSprockets("encabulators")
                    Ω(sprockets).Should(BeEmpty())
                    Ω(err).Should(MatchError(SprocketsErrorUnauthorized))
                })
            })

            Context("when the response is not found", func() {
                BeforeEach(func() {
                    statusCode = http.StatusNotFound
                })

                It("should return the SprocketsErrorNotFound error", func() {
                    sprockets, err := client.FetchSprockets("encabulators")
                    Ω(sprockets).Should(BeEmpty())
                    Ω(err).Should(MatchError(SprocketsErrorNotFound))
                })
            })
        })
    })

In this way, the status code and returned value (not shown here) can be changed in sub-contexts without having to modify the original test setup.

Handling multiple requests

So far, we’ve only seen examples where one request is made per test. ghttp supports handling multiple requests too. server.AppendHandlers can be passed multiple handlers and these handlers are evaluated in order as requests come in.

This can be helpful in cases where it is not possible (or desirable) to have calls to the client under test only generate one request. A common example is pagination. If the sprockets API is paginated it may be desirable for FetchSprockets to provide a simpler interface that simply fetches all available sprockets.

Here’s what a test might look like:

    Describe("fetching sprockets from a paginated endpoint", func() {
        var returnedSprockets []Sprocket
        var firstResponse, secondResponse PaginatedResponse

        BeforeEach(func() {
            returnedSprockets = []Sprocket{
                sprockets.Sprocket{Name: "entropic decoupler", Color: "red"},
                sprockets.Sprocket{Name: "defragmenting ramjet", Color: "yellow"},
                sprockets.Sprocket{Name: "parametric demuxer", Color: "blue"},
            }

            firstResponse = sprockets.PaginatedResponse{
                Sprockets: returnedSprockets[0:2], //first batch
                PaginationToken: "get-second-batch", //some opaque non-empty token
            }

            secondResponse = sprockets.PaginatedResponse{
                Sprockets: returnedSprockets[2:], //second batch
                PaginationToken: "", //signifies the last batch
            }

            server.AppendHandlers(
                ghttp.CombineHandlers(
                    ghttp.VerifyRequest("GET", "/sprockets", "category=encabulators"),
                    ghttp.RespondWithJSONEncoded(http.StatusOK, firstResponse),
                ),
                ghttp.CombineHandlers(
                    ghttp.VerifyRequest("GET", "/sprockets", "category=encabulators&pagination-token=get-second-batch"),
                    ghttp.RespondWithJSONEncoded(http.StatusOK, secondResponse),
                ),
            )
        })

        It("should fetch all the sprockets", func() {
            sprockets, err := client.FetchSprockets("encabulators")
            Ω(err).ShouldNot(HaveOccurred())
            Ω(sprockets).Should(Equal(returnedSprockets))
        })
    })

By default the ghttp server fails the test if the number of requests received exceeds the number of handlers registered, so this test ensures that the client stops sending requests after receiving the second (and final) set of paginated data.

MUXing Routes to Handlers

AppendHandlers allows you to make ordered assertions about incoming requests. This places a strong constraint on all incoming requests: namely that exactly the right requests have to arrive in exactly the right order and that no additional requests are allowed.

One can take a different testing strategy, however. Instead of asserting that requests come in in a predefined order, you may which to build a test server that can handle arbitrarily many requests to a set of predefined routes. In fact, there may be some circumstances where you want to make ordered assertions on some requests (via AppendHandlers) but still support sending particular responses to other requests that may interleave the ordered assertions.

ghttp supports these sorts of usecases via server.RouteToHandler(method, path, handler).

Let’s cook up an example. Perhaps, instead of authenticating via basic auth our sprockets client logs in and fetches a token from the server when performing requests that require authentication. We could pepper our AppendHandlers calls with a handler that handles these requests (this is not a terrible idea, of course!) or we could set up a single route at the top of our tests.

Here’s what such a test might look like:

    Describe("CRUDing sprockes", func() {
        BeforeEach(func() {
            server.RouteToHandler("POST", "/login", ghttp.CombineHandlers(
                ghttp.VerifyRequest("POST", "/login", "user=bob&password=password"),
                ghttp.RespondWith(http.StatusOK, "your-auth-token"),
            ))
        })
        Context("GETting sprockets", func() {
            var returnedSprockets []Sprocket

            BeforeEach(func() {
                returnedSprockets = []Sprocket{
                    sprockets.Sprocket{Name: "entropic decoupler", Color: "red"},
                    sprockets.Sprocket{Name: "defragmenting ramjet", Color: "yellow"},
                    sprockets.Sprocket{Name: "parametric demuxer", Color: "blue"},
                }

                server.AppendHandlers(
                    ghttp.CombineHandlers(
                        ghttp.VerifyRequest("GET", "/sprockets", "category=encabulators"),
                        ghttp.RespondWithJSONEncoded(http.StatusOK, returnedSprockets),
                    ),
                )
            })

            It("should fetch all the sprockets", func() {
                sprockets, err := client.FetchSprockes("encabulators")
                Ω(err).ShouldNot(HaveOccurred())
                Ω(sprockets).Should(Equal(returnedSprockets))
            })
        })

        Context("POSTing sprockets", func() {
            var sprocketToSave Sprocket
            BeforeEach(func() {
                sprocketToSave = sprockets.Sprocket{Name: "endothermic penambulator", Color: "purple"}

                server.AppendHandlers(
                    ghttp.CombineHandlers(
                        ghttp.VerifyRequest("POST", "/sprocket", "token=your-auth-token"),
                        ghttp.VerifyJSONRepresenting(sprocketToSave)
                        ghttp.RespondWithJSONEncoded(http.StatusOK, nil),
                    ),
                )
            })

            It("should save the sprocket", func() {
                err := client.SaveSprocket(sprocketToSave)
                Ω(err).ShouldNot(HaveOccurred())
            })
        })
    })

Here, saving a sprocket triggers authentication, which is handled by the registered RouteToHandler handler whereas fetching the list of sprockets does not.

RouteToHandler can take either a string as a route (as seen in this example) or a regexp.Regexp.

Allowing unhandled requests

By default, ghttp’s server marks the test as failed if a request is made for which there is no registered handler.

It is sometimes useful to have a fake server that simply returns a fixed status code for all unhandled incoming requests. ghttp supports this: just call server.SetAllowUnhandledRequests(true) and server.SetUnhandledRequestStatusCode(statusCode), passing whatever status code you’d like to return.

In addition to returning the registered status code, ghttp’s server will also save all received requests. These can be accessed by calling server.ReceivedRequests(). This is useful for cases where you may want to make assertions against requests after they’ve been made.

To bring it all together: there are three ways to instruct a ghttp server to handle requests: you can map routes to handlers using RouteToHandler, you can append handlers via AppendHandlers, and you can SetAllowUnhandledRequests and specify the status code by calling SetUnhandledRequestStatusCode.

When a ghttp server receives a request it first checks against the set of handlers registered via RouteToHandler if there is no such handler it proceeds to pop an AppendHandlers handler off the stack, if the stack of ordered handlers is empty, it will check whether GetAllowUnhandledRequests returns true or false. If false the test fails. If true, a response is sent with whatever GetUnhandledRequestStatusCode returns.

gbytes: Testing Streaming Buffers

gbytes implements gbytes.Buffer - an io.WriteCloser that captures all input to an in-memory buffer.

When used in concert with the gbytes.Say matcher, the gbytes.Buffer allows you make ordered assertions against streaming data.

What follows is a contrived example. gbytes is best paired with gexec.

Say you have an integration test that is streaming output from an external API. You can feed this stream into a gbytes.Buffer and make ordered assertions like so:

    Describe("attach to the data stream", func() {
        var (
            client *apiclient.Client
            buffer *gbytes.Buffer
        )
        BeforeEach(func() {
            buffer = gbytes.NewBuffer()
            client := apiclient.New()
            go client.AttachToDataStream(buffer)
        })

        It("should stream data", func() {
            Eventually(buffer).Should(gbytes.Say(`Attached to stream as client \d+`))

            client.ReticulateSplines()
            Eventually(buffer).Should(gbytes.Say(`reticulating splines`))
            client.EncabulateRetros(7)
            Eventually(buffer).Should(gbytes.Say(`encabulating 7 retros`))
        })
    })

These assertions will only pass if the strings passed to Say (which are interpreted as regular expressions - make sure to escape characters appropriately!) appear in the buffer. An opaque read cursor (that you cannot access or modify) is fast-forwarded as successful assertions are made. So, for example:

    Eventually(buffer).Should(gbytes.Say(`reticulating splines`))
    Consistently(buffer).ShouldNot(gbytes.Say(`reticulating splines`))

will (counterintuitively) pass. This allows you to write tests like:

    client.ReticulateSplines()
    Eventually(buffer).Should(gbytes.Say(`reticulating splines`))
    client.ReticulateSplines()
    Eventually(buffer).Should(gbytes.Say(`reticulating splines`))

and ensure that the test is correctly asserting that reticulating splines appears twice.

At any time, you can access the entire contents written to the buffer via buffer.Contents(). This includes everything ever written to the buffer regardless of the current position of the read cursor.

Handling branches

Sometimes (rarely!) you must write a test that must perform different actions depending on the output streamed to the buffer. This can be accomplished using buffer.Detect. Here’s a contrived example:

    func LoginIfNecessary() {
        client.Authorize()
        select {
        case <-buffer.Detect("You are not logged in"):
            client.Login()
        case <-buffer.Detect("Success"):
            return
        case <-time.After(time.Second):
            ginkgo.Fail("timed out waiting for output")
        }
        buffer.CancelDetects()
    }

buffer.Detect takes a string (interpreted as a regular expression) and returns a channel that will fire once if the requested string is detected. Upon detection, the buffer’s opaque read cursor is fast-forwarded so subsequent uses of gbytes.Say will pick up from where the succeeding Detect left off. You must call buffer.CancelDetects() to clean up afterwards (buffer spawns one goroutine per call to Detect).

Testing io.Readers, io.Writers, and io.Closers

Implementations of io.Reader, io.Writer, and io.Closer are expected to be blocking. This makes the following class of tests unsafe:

    It("should read something", func() {
        p := make([]byte, 5)
        _, err := reader.Read(p)  //unsafe! this could block forever
        Ω(err).ShouldNot(HaveOccurred())
        Ω(p).Should(Equal([]byte("abcde")))
    })

It is safer to wrap io.Readers, io.Writers, and io.Closers with explicit timeouts. You can do this with gbytes.TimeoutReader, gbytes.TimeoutWriter, and gbytes.TimeoutCloser like so:

    It("should read something", func() {
        p := make([]byte, 5)
        _, err := gbytes.TimeoutReader(reader, time.Second).Read(p)
        Ω(err).ShouldNot(HaveOccurred())
        Ω(p).Should(Equal([]byte("abcde")))
    })

The gbytes wrappers will return gbytes.ErrTimeout if a timeout occurs.

In the case of io.Readers you can leverage the Say matcher and the functionality of gbytes.Buffer by building a gbytes.Buffer that reads from the io.Reader asynchronously. You can do this with the gbytes package like so:

    It("should read something", func() {
        Eventually(gbytes.BufferReader(reader)).Should(gbytes.Say("abcde"))
    })

gbytes.BufferReader takes an io.Reader and returns a gbytes.Buffer. Under the hood an io.Copy goroutine is launched to copy data from the io.Reader into the gbytes.Buffer. The gbytes.Buffer is closed when the io.Copy completes. Because the io.Copy is launched asynchronously you must make assertions against the reader using Eventually.

gexec: Testing External Processes

gexec simplifies testing external processes. It can help you compile go binaries, start external processes, send signals and wait for them to exit, make assertions against the exit code, and stream output into gbytes.Buffers to allow you make assertions against output.

Compiling external binaries

You use gexec.Build() to compile Go binaries. These are built using go build and are stored off in a temporary directory. You’ll want to gexec.CleanupBuildArtifacts() when you’re done with the test.

A common pattern is to compile binaries once at the beginning of the test using BeforeSuite and to clean up once at the end of the test using AfterSuite:

    var pathToSprocketCLI string

    BeforeSuite(func() {
        var err error
        pathToSprocketCLI, err = gexec.Build("github.com/spacely/sprockets")
        Ω(err).ShouldNot(HaveOccurred())
    })

    AfterSuite(func() {
        gexec.CleanupBuildArtifacts()
    })

By default, gexec.Build uses the GOPATH specified in your environment. You can also use gexec.BuildIn(gopath string, packagePath string) to specify a custom GOPATH for the build command. This is useful to, for example, build a binary against its vendored Godeps.

You can specify arbitrary environment variables for the build command – such as GOOS and GOARCH for building on other platforms – using gexec.BuildWithEnvironment(packagePath string, envs []string).

Starting external processes

gexec provides a Session that wraps exec.Cmd. Session includes a number of features that will be explored in the next few sections. You create a Session by instructing gexec to start a command:

    command := exec.Command(pathToSprocketCLI, "-api=127.0.0.1:8899")
    session, err := gexec.Start(command, GinkgoWriter, GinkgoWriter)
    Ω(err).ShouldNot(HaveOccurred())

gexec.Start calls command.Start for you and forwards the command’s stdout and stderr to io.Writers that you provide. In the code above, we pass in Ginkgo’s GinkgoWriter. This makes working with external processes quite convenient: when a test passes no output is printed to screen, however if a test fails then any output generated by the command will be provided.

If you want to see all your output regardless of test status, just run ginkgo in verbose mode (-v) - now everything written to GinkgoWriter makes it onto the screen.

Sending signals and waiting for the process to exit

gexec.Session makes it easy to send signals to your started command:

    session.Kill() //sends SIGKILL
    session.Interrupt() //sends SIGINT
    session.Terminate() //sends SIGTERM
    session.Signal(signal) //sends the passed in os.Signal signal

If the process has already exited these signal calls are no-ops.

In addition to starting the wrapped command, gexec.Session also monitors the command until it exits. You can ask gexec.Session to Wait until the process exits:

    session.Wait()

this will block until the session exits and will fail if it does not exit within the default Eventually timeout. You can override this timeout by specifying a custom one:

    session.Wait(5 * time.Second)

Though you can access the wrapped command using session.Command you should not attempt to Wait on it yourself. gexec has already called Wait in order to monitor your process for you.

Under the hood session.Wait simply uses Eventually.

Since the signalling methods return the session you can chain calls together:

    session.Terminate().Wait()

will send SIGTERM and then wait for the process to exit.

Asserting against exit code

Once a session has exited you can fetch its exit code with session.ExitCode(). You can subsequently make assertions against the exit code.

A more idiomatic way to assert that a command has exited is to use the gexec.Exit() matcher:

    Eventually(session).Should(Exit())

Will verify that the session exits within Eventually’s default timeout. You can assert that the process exits with a specified exit code too:

    Eventually(session).Should(Exit(0))

If the process has not exited yet, session.ExitCode() returns -1

Making assertions against the process output

In addition to streaming output to the passed in io.Writers (the GinkgoWriter in our example above), gexec.Start attaches gbytes.Buffers to the command’s output streams. These are available on the session object via:

    session.Out //a gbytes.Buffer connected to the command's stdout
    session.Err //a gbytes.Buffer connected to the command's stderr

This allows you to make assertions against the stream of output:

    Eventually(session.Out).Should(gbytes.Say("hello [A-Za-z], nice to meet you"))
    Eventually(session.Err).Should(gbytes.Say("oops!"))

Since gexec.Session is a gbytes.BufferProvider that provides the Out buffer you can write assertions against stdout output like so:

    Eventually(session).Should(gbytes.Say("hello [A-Za-z], nice to meet you"))

Using the Say matcher is convenient when making ordered assertions against a stream of data generated by a live process. Sometimes, however, all you need is to wait for the process to exit and then make assertions against the entire contents of its output. Since Wait() returns session you can wait for the process to exit, then grab all its stdout as a []byte buffer with a simple one-liner:

    Ω(session.Wait().Out.Contents()).Should(ContainSubstring("finished successfully"))

Signaling all processes

gexec provides methods to track and send signals to all processes that it starts.

    gexec.Kill() //sends SIGKILL to all processes
    gexec.Terminate() //sends SIGTERM to all processes
    gexec.Signal(int)  //sends the passed in os.Signal signal to all the processes
    gexec.Interrupt() //sends SIGINT to all processes

If the any of the processes have already exited these signal calls are no-ops.

gexec also provides methods to cleanup and wait for all the processes it started.

    gexec.KillAndWait()
    gexec.TerminateAndWait()

You can specify a custom timeout by:

    gexec.KillAndWait(5 * time.Second)
    gexec.TerminateAndWait(2 * time.Second)

The timeout is applied for each of the processes.

It is considered good practice to ensure all of your processes have been killed before the end of the test suite. If you are using ginkgo you can use:

    AfterSuite(func(){
        gexec.KillAndWait()
    })

Due to the global nature of these methods, keep in mind that signaling processes will affect all processes started by gexec, in any context. For example if these methods where used in an AfterEach, then processes started in BeforeSuite would also be signaled.

gstruct: Testing Complex Data Types

gstruct simplifies testing large and nested structs and slices. It is used for building up complex matchers that apply different tests to each field or element.

Testing type struct

gstruct provides the FieldsMatcher through the MatchAllFields and MatchFields functions for applying a separate matcher to each field of a struct:

    actual := struct{
        A int
        B bool
        C string
    }{5, true, "foo"}
    Expect(actual).To(MatchAllFields(Fields{
        "A": BeNumerically("<", 10),
        "B": BeTrue(),
        "C": Equal("foo"),
    }))

MatchAllFields requires that every field is matched, and each matcher is mapped to a field. To match a subset or superset of a struct, you should use the MatchFields function with the IgnoreExtras and IgnoreMissing options. IgnoreExtras will ignore fields that don’t map to a matcher, e.g.

    Expect(actual).To(MatchFields(IgnoreExtras, Fields{
        "A": BeNumerically("<", 10),
        "B": BeTrue(),
        // Ignore lack of "C" in the matcher.
    }))

IgnoreMissing will ignore matchers that don’t map to a field, e.g.

    Expect(actual).To(MatchFields(IgnoreMissing, Fields{
        "A": BeNumerically("<", 10),
        "B": BeTrue(),
        "C": Equal("foo"),
        "D": Equal("bar"), // Ignored, since actual.D does not exist.
    }))

The options can be combined with the binary or: IgnoreMissing|IgnoreExtras.

Testing type slice

gstruct provides the ElementsMatcher through the MatchAllElements and MatchElements function for applying a separate matcher to each element, identified by an Identifier function:

    actual := []string{
        "A: foo bar baz",
        "B: once upon a time",
        "C: the end",
    }
    id := func(element interface{}) string {
        return string(element.(string)[0])
    }
    Expect(actual).To(MatchAllElements(id, Elements{
        "A": Not(BeZero()),
        "B": MatchRegexp("[A-Z]: [a-z ]+"),
        "C": ContainSubstring("end"),
    }))

MatchAllElements requires that there is a 1:1 mapping from every element to every matcher. To match a subset or superset of elements, you should use the MatchElements function with the IgnoreExtras and IgnoreMissing options. IgnoreExtras will ignore elements that don’t map to a matcher, e.g.

    Expect(actual).To(MatchElements(id, IgnoreExtras, Elements{
        "A": Not(BeZero()),
        "B": MatchRegexp("[A-Z]: [a-z ]+"),
        // Ignore lack of "C" in the matcher.
    }))

IgnoreMissing will ignore matchers that don’t map to an element, e.g.

    Expect(actual).To(MatchElements(id, IgnoreMissing, Elements{
        "A": Not(BeZero()),
        "B": MatchRegexp("[A-Z]: [a-z ]+"),
        "C": ContainSubstring("end"),
        "D": Equal("bar"), // Ignored, since actual.D does not exist.
    }))

You can also use the flag AllowDuplicates to permit multiple elements in your slice to map to a single key and matcher in your fields (this flag is not meaningful when applied to structs).

    everyElementID := func(element interface{}) string {
        return "a constant" // Every element will map to the same key in this case; you can group them into multiple keys, however.
    }
    Expect(actual).To(MatchElements(everyElementID, AllowDuplicates, Elements{
        "a constant": ContainSubstring(": "), // Because every element passes this test
    }))
    Expect(actual).NotTo(MatchElements(everyElementID, AllowDuplicates, Elements{
        "a constant": ContainSubstring("foo bar baz"), // Only the first element passes this test
    }))

The options can be combined with the binary or: IgnoreMissing|IgnoreExtras|AllowDuplicates.

Additionally, gstruct provides MatchAllElementsWithIndex and MatchElementsWithIndex function for applying a matcher with index to each element, identified by an IdentifierWithIndex function. A helper function is also included with gstruct called IndexIdentity that provides the functionality of the just using the index as your identifier as seen below.

    actual := []string{
        "A: foo bar baz",
        "B: once upon a time",
        "C: the end",
    }
    id := func(index int, _ interface{}) string {
        return strconv.Itoa(index)
    }
    Expect(actual).To(MatchAllElementsWithIndex(id, Elements{
        "0": Not(BeZero()),
        "1": MatchRegexp("[A-Z]: [a-z ]+"),
        "2": ContainSubstring("end"),
    }))
    // IndexIdentity is a helper function equivalent to id in this example
    Expect(actual).To(MatchAllElementsWithIndex(IndexIdentity, Elements{
        "0": Not(BeZero()),
        "1": MatchRegexp("[A-Z]: [a-z ]+"),
        "2": ContainSubstring("end"),
    }))

The WithIndex variants take the same options as the other functions.

Testing type map

All of the *Fields functions and types have a corresponding definitions *Keys which can perform analogous tests against map types:

    actual := map[string]string{
        "A": "correct",
        "B": "incorrect",
    }

    // fails, because `actual` includes the key B
    Expect(actual).To(MatchAllKeys(Keys{
        "A": Equal("correct"),
    }))

    // passes
    Expect(actual).To(MatchAllKeys(Keys{
        "A": Equal("correct"),
        "B": Equal("incorrect"),
    }))

    // passes
    Expect(actual).To(MatchKeys(IgnoreMissing, Keys{
        "A": Equal("correct"),
        "B": Equal("incorrect"),
        "C": Equal("whatever"), // ignored, because `actual` doesn't have this key
    }))

Testing pointer values

gstruct provides the PointTo function to apply a matcher to the value pointed-to. It will fail if the pointer value is nil: