Faking a dependency in Go

Depend on an interface and the test supplies its own implementation.

type Store interface {
    Save(lang string, tpm int) error
}

func Record(store Store, lang string, tpm int) error {
    if tpm <= 0 {
        return errors.New("tpm must be positive")
    }
    return store.Save(lang, tpm)
}

type fakeStore struct {
    calls []string
    err   error
}

func (f *fakeStore) Save(lang string, tpm int) error {
    f.calls = append(f.calls, fmt.Sprintf("%s=%d", lang, tpm))
    return f.err
}

func TestRecord(t *testing.T) {
    fake := &fakeStore{}
    if err := Record(fake, "go", 104); err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    if len(fake.calls) != 1 || fake.calls[0] != "go=104" {
        t.Errorf("calls = %v", fake.calls)
    }
}

How it works

1. The production code never mentions the fake.
2. One small interface is easier to fake than a big one.
3. Recording calls in the fake is how you assert on them.

Keywords and builtins used here

• append
• error
• func
• if
• int
• interface
• len
• return
• string
• struct
• type

The run, in numbers

Lines

30

Characters to type

622

Tokens

176

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 101 seconds.

Type this snippet

Step 1 of 3 in Fakes & fixtures, step 5 of 13 in Testing.

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