How do you combine table-driven tests with interface mocks in Go?

The two most important Go testing patterns together: table-driven tests for coverage and interface mocks for isolation. Combining them gives thorough, readable, and maintainable tests for any component that has external dependencies.

// System under test
type UserEmailer interface {
    GetEmail(userID int) (string, error)
}

type Mailer interface {
    Send(to, subject, body string) error
}

type NotificationService struct {
    emailer UserEmailer
    mailer  Mailer
}

func (ns *NotificationService) Notify(userID int, msg string) error {
    email, err := ns.emailer.GetEmail(userID)
    if err != nil { return fmt.Errorf("get email: %w", err) }
    return ns.mailer.Send(email, "Notification", msg)
}

// Test mocks
type fakeEmailer struct{ email string; err error }
func (f *fakeEmailer) GetEmail(int) (string, error) { return f.email, f.err }

type fakeMailer struct{ sent []string; err error }
func (m *fakeMailer) Send(to, _, _ string) error {
    m.sent = append(m.sent, to)
    return m.err
}

// Table-driven test combining both mocks
func TestNotify(t *testing.T) {
    tests := []struct {
        name      string
        emailErr  error
        mailerErr error
        wantErr   bool
        wantSent  int
    }{
        {"happy path", nil, nil, false, 1},
        {"email lookup fails", errors.New("db error"), nil, true, 0},
        {"mailer fails", nil, errors.New("smtp error"), true, 0},
    }
    for _, tc := range tests {
        t.Run(tc.name, func(t *testing.T) {
            mailer := &fakeMailer{err: tc.mailerErr}
            svc := &NotificationService{
                emailer: &fakeEmailer{email: "u@x.com", err: tc.emailErr},
                mailer:  mailer,
            }
            err := svc.Notify(1, "hello")
            if (err != nil) != tc.wantErr {
                t.Errorf("want err=%v, got %v", tc.wantErr, err)
            }
            if len(mailer.sent) != tc.wantSent {
                t.Errorf("want %d sent, got %d", tc.wantSent, len(mailer.sent))
            }
        })
    }
}

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