Inject test doubles through the Factory; never monkey-patch globals

byob-testing.1 testing

Problem: tests that capture command output by reassigning os.Stdout, or replace an HTTP client by overwriting a package-level var httpClient = ..., or use monkey.Patch to rewire function pointers — all of these are flaky, non-parallel-safe, and break the moment another test runs concurrently. They also leak state between tests and make it impossible to know what the code under test actually does.

Idea: every swappable dependency on the Factory is either an interface (Prompter, Store, Browser, HTTP client) or an IOStreams value. Tests construct a Factory literal with purpose-built fakes for the interfaces, and with iostreams.Test() for the streams. The command under test gets the test Factory via its constructor. No globals are touched anywhere.

iostreams.Test() returns an IOStreams wired to bytes.Buffer values plus the buffers themselves, so the test can assert on stdout/stderr contents after running the command. Interface fakes are tiny (often <50 lines) and can record calls, script canned responses, or both.

Tradeoffs: you write the fakes. For high-churn or wide interfaces, code generators (mockery, counterfeiter) pay off; for narrow interfaces used by a few tests, hand-rolling stays clearer. TTY-dependent code paths need explicit SetStdoutTTY(true) calls on the test IOStreams — that's a feature, not a bug (TTY behavior should be exercised deliberately).

When not to use: never. The principle — "test doubles are constructor arguments, not package variables" — applies universally.

Design

// iostreams.Test() returns buffers for output assertions.
func Test() (*IOStreams, *bytes.Buffer, *bytes.Buffer, *bytes.Buffer) {
    in, out, errOut := &bytes.Buffer{}, &bytes.Buffer{}, &bytes.Buffer{}
    return &IOStreams{
        In: in, Out: out, ErrOut: errOut,
    }, in, out, errOut
}

// Interface fakes record calls and script replies.
type fakePrompter struct {
    confirmReply bool
    confirmCalls []string
}
func (f *fakePrompter) Confirm(msg string) (bool, error) {
    f.confirmCalls = append(f.confirmCalls, msg)
    return f.confirmReply, nil
}

// A whole test:
func TestListDefault(t *testing.T) {
    io, _, stdout, stderr := iostreams.Test()
    p := &fakePrompter{confirmReply: true}
    f := &Factory{IOStreams: io, Prompter: p, Store: fakeStore}

    cmd := NewCmdList(f, nil)
    cmd.SetArgs([]string{"--format", "tsv"})
    require.NoError(t, cmd.Execute())
    require.Contains(t, stdout.String(), "item-1\titem-2")
    require.Empty(t, stderr.String())
}