サンプル: Golangにおけるinterfaceをつかったテストで mock を書く技法

main.go

package main

import (
	"context"
	"fmt"
)

type GitHub interface {
	CreateRelease(ctx context.Context, opt *Option) (string, error)
	GetRelease(ctx context.Context, tag string) (string, error)
	DeleteRelease(ctx context.Context, releaseID int) error
}

type GhRelease struct {
	c GitHub
}

func (ghr *GhRelease) CreateNewRelease(ctx context.Context) (*Release, error) {
	tag, err := ghr.c.CreateRelease(ctx, nil)
	if err != nil {
		return nil, fmt.Errorf("failed to create release: %v", err)
	}

	// check created release
	if _, err := ghr.c.GetRelease(ctx, tag); err != nil {
		return nil, fmt.Errorf("failed to get created release: %v", err)
	}

	// ...
	return &Release{}, nil
}

type Option struct{}
type Release struct{}

main_test.go

package main

import (
	"context"
	"fmt"
	"testing"
)

type fakeGitHub struct {
	// インターフェース埋め込み
	GitHub
	FakeCreateRelease func(ctx context.Context, opt *Option) (string, error)
	FakeGetRelease    func(ctx context.Context, tag string) (string, error)
	// 埋め込みを使うので，例えば DeleteRelease はまだテストしないので mock
	// しない... いうことができる．
}

func (c *fakeGitHub) CreateRelease(ctx context.Context, opt *Option) (string, error) {
	return c.FakeCreateRelease(ctx, opt)
}

func (c *fakeGitHub) GetRelease(ctx context.Context, tag string) (string, error) {
	return c.FakeGetRelease(ctx, tag)
}

func TestGhRelease_CreateNewRelease(t *testing.T) {
	fakeclient := &fakeGitHub{
		FakeCreateRelease: func(ctx context.Context, opt *Option) (string, error) {
			return "v1.0", nil
		},
		FakeGetRelease: func(ctx context.Context, tag string) (string, error) {
			return "", fmt.Errorf("failed to get %v release!", tag)
		},
	}

	ghr := &GhRelease{c: fakeclient}

	release, err := ghr.CreateNewRelease(context.Background())
	if err != nil {
		t.Error(err)
		// => failed to get created release: failed to get v1.0 release!
	}
	_ = release
	// ...
}

とても参考になりました！