When writing unit-tests for an object, it is often useful to have fake implementations of the object's collaborators. In go, such fake implementations cannot be generated automatically at runtime, and writing them by hand can be quite arduous.

counterfeiter allows you to simply generate test doubles for a given interface.

counterfeiter follows the support policy of go itself:

Each major Go release is supported until there are two newer major releases. For example, Go 1.5 was supported until the Go 1.7 release, and Go 1.6 was supported until the Go 1.8 release. We fix critical problems, including critical security problems, in supported releases as needed by issuing minor revisions (for example, Go 1.6.1, Go 1.6.2, and so on).

If you are having problems with counterfeiter and are not using a supported version of go, please update to use a supported version of go before opening an issue.

⚠️ Please use go modules when working with counterfeiter.

Typically, counterfeiter is used in go generate directives. It can be frustrating when you change your interface declaration and suddenly all of your generated code is suddenly out-of-date. The best practice here is to use the go generate command to make it easier to keep your test doubles up to date.

⚠️ If you are working with go 1.23 or earlier, please refer to an older version of this README, as the instructions below assume go 1.24 (which added go tool support) and later.

Establish a tool dependency on counterfeiter by running the following command:

go get -tool github.com/maxbrunsfeld/counterfeiter/v6

You can add directives right next to your interface definitions (or not), in any .go file in your module.

$ cat myinterface.go

package foo

//go:generate go tool counterfeiter . MySpecialInterface

type MySpecialInterface interface {
	DoThings(string, uint64) (int, error)
}

$ go generate ./...
Writing `FakeMySpecialInterface` to `foofakes/fake_my_special_interface.go`... Done

If you plan to have many directives in a single package, consider using this option, as it will speed things up considerably. You can add directives right next to your interface definitions (or not), in any .go file in your module.

$ cat myinterface.go

package foo

// You only need **one** of these per package!
//go:generate go tool counterfeiter -generate

// You will add lots of directives like these in the same package...
//counterfeiter:generate . MySpecialInterface
type MySpecialInterface interface {
	DoThings(string, uint64) (int, error)
}

// Like this...
//counterfeiter:generate . MyOtherInterface
type MyOtherInterface interface {
	DoOtherThings(string, uint64) (int, error)
}

$ go generate ./...
Writing `FakeMySpecialInterface` to `foofakes/fake_my_special_interface.go`... Done
Writing `FakeMyOtherInterface` to `foofakes/fake_my_other_interface.go`... Done

You can run go generate in the directory with your directive, or in the root of your module (to ensure you generate for all packages in your module):

$ go generate ./...

You can use the following command to invoke counterfeiter from within a go module:

$ go tool counterfeiter

USAGE
	counterfeiter
		[-generate>] [-o <output-path>] [-p] [--fake-name <fake-name>]
		[-header <header-file>]
		[<source-path>] <interface> [-]

This is unnecessary if you're using the approach described above, but does allow you to invoke counterfeiter in your shell outside of a module:

$ go install github.com/maxbrunsfeld/counterfeiter/v6
$ ~/go/bin/counterfeiter

USAGE
	counterfeiter
		[-generate>] [-o <output-path>] [-p] [--fake-name <fake-name>]
		[-header <header-file>]
		[<source-path>] <interface> [-]

Given a path to a package and an interface name, you can generate a test double.

$ cat path/to/foo/file.go

package foo

type MySpecialInterface interface {
		DoThings(string, uint64) (int, error)
}

$ go tool counterfeiter path/to/foo MySpecialInterface
Wrote `FakeMySpecialInterface` to `path/to/foo/foofakes/fake_my_special_interface.go`

Instantiate fakes:

import "my-repo/path/to/foo/foofakes"

var fake = &foofakes.FakeMySpecialInterface{}

Fakes record the arguments they were called with:

fake.DoThings("stuff", 5)

Expect(fake.DoThingsCallCount()).To(Equal(1))

str, num := fake.DoThingsArgsForCall(0)
Expect(str).To(Equal("stuff"))
Expect(num).To(Equal(uint64(5)))

You can stub their return values:

fake.DoThingsReturns(3, errors.New("the-error"))

num, err := fake.DoThings("stuff", 5)
Expect(num).To(Equal(3))
Expect(err).To(Equal(errors.New("the-error")))

For more examples of using the counterfeiter API, look at some of the provided examples.

For third party interfaces, you can specify the interface using the alternative syntax <package>.<interface>, for example:

$ go tool counterfeiter github.com/go-redis/redis.Pipeliner

If you want to run the tests for counterfeiter (perhaps, because you want to contribute a PR), all you have to do is run scripts/ci.sh.

So you want to contribute to counterfeiter! That's great, here's exactly what you should do:

• open a new github issue, describing your problem, or use case
• help us understand how you want to fix or extend counterfeiter
• write one or more unit tests for the behavior you want
• write the simplest code you can for the feature you're working on
• try to find any opportunities to refactor
• avoid writing code that isn't covered by unit tests

counterfeiter has a few high level goals for contributors to keep in mind

• keep unit-level test coverage as high as possible
• keep main.go as simple as possible
• avoid making the command line options any more complicated
• avoid making the internals of counterfeiter any more complicated

If you have any questions about how to contribute, rest assured that @tjarratt and other maintainers will work with you to ensure we make counterfeiter better, together. This project has largely been maintained by the community, and we greatly appreciate any PR (whether big or small).

counterfeiter is MIT-licensed.