mockery provides the ability to easily generate mocks for golang interfaces. It removes the boilerplate coding required to use mocks.

go get github.com/vektra/mockery/.../, then $GOPATH/bin/mockery

Given this is in string.go

package test

type Stringer interface {
	String() string
}

Run: mockery -name=Stringer and the following will be output to mocks/Stringer.go:

package mocks

import "github.com/stretchr/testify/mock"

type Stringer struct {
	mock.Mock
}

func (m *Stringer) String() string {
	ret := m.Called()

	var r0 string
	if rf, ok := ret.Get(0).(func() string); ok {
		r0 = rf()
	} else {
		r0 = ret.Get(0).(string)
	}

	return r0
}

See github.com/jaytaylor/mockery-example for the fully runnable version of the outline below.

package main

import (
	"fmt"

	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/service/s3"
	"github.com/jaytaylor/mockery-example/mocks"
	"github.com/stretchr/testify/mock"
)

func main() {
	mockS3 := &mocks.S3API{}

	mockResultFn := func(input *s3.ListObjectsInput) *s3.ListObjectsOutput {
		output := &s3.ListObjectsOutput{}
		output.SetCommonPrefixes([]*s3.CommonPrefix{
			&s3.CommonPrefix{
				Prefix: aws.String("2017-01-01"),
			},
		})
		return output
	}

	// NB: .Return(...) must return the same signature as the method being mocked.
	//     In this case it's (*s3.ListObjectsOutput, error).
	mockS3.On("ListObjects", mock.MatchedBy(func(input *s3.ListObjectsInput) bool {
		return input.Delimiter != nil && *input.Delimiter == "/" && input.Prefix == nil
	})).Return(mockResultFn, nil)

	listingInput := &s3.ListObjectsInput{
		Bucket:    aws.String("foo"),
		Delimiter: aws.String("/"),
	}
	listingOutput, err := mockS3.ListObjects(listingInput)
	if err != nil {
		panic(err)
	}

	for _, x := range listingOutput.CommonPrefixes {
		fmt.Printf("common prefix: %+v\n", *x)
	}
}

mockery pulls in all the same imports used in the file that contains the interface so that package types will work correctly. It then runs the output through the imports package to remove any unnecessary imports (as they'd result in compile errors).

mockery should handle all types. If you find it does not, please report the issue.

If your tests need access to the arguments to calculate the return values, set the return value to a function that takes the method's arguments as its own arguments and returns the return value. For example, given this interface:

package test

type Proxy interface {
  passthrough(s string) string
}

The argument can be passed through as the return value:

import . "github.com/stretchr/testify/mock"

Mock.On("passthrough", AnythingOfType("string")).Return(func(s string) string {
    return s
})

Return must be passed the same argument count and types as expected by the interface. If the return argument signature of passthrough in the above example was instead (string, error) in the interface, Return would also need a second argument to define the error value.

If any return argument is missing, github.com/stretchr/testify/mock.Arguments.Get will emit a panic.

For example, panic: assert: arguments: Cannot call Get(0) because there are 0 argument(s). [recovered] indicates that Return was not provided any arguments but (at least one) was expected based on the interface. Get(1) would indicate that the Return call is missing a second argument, and so on.

This approach should be used judiciously, as return values should generally not depend on arguments in mocks; however, this approach can be helpful for situations like passthroughs or other test-only calculations.

The -name option takes either the name or matching regular expression of interface to generate mock(s) for.

It's common for a big package to have a lot of interfaces, so mockery provides -all. This option will tell mockery to scan all files under the directory named by -dir ("." by default) and generates mocks for any interfaces it finds. This option implies -recursive=true.

-all was designed to be able to be used automatically in the background if required.

Use the -recursive option to search subdirectories for the interface(s). This option is only compatible with -name. The -all option implies -recursive=true.

mockery always generates files with the package mocks to keep things clean and simple. You can control which mocks directory is used by using -output, which defaults to ./mocks.

mockery generates files using the caseing of the original interface name. This can be modified by specifying -case=underscore to format the generated file name using underscore casing.

Use mockery -print to have the resulting code printed out instead of written to disk.

When your interfaces are in the main package you should supply the -inpkg flag. This will generate mocks in the same package as the target code avoiding import issues.