logtest

Loggers for tests.

log := logtest.New(t) // every record, at debug and above, into t.Log

A failing test then prints what the code under test logged; a passing one prints nothing unless run with -v.

log, rec := logtest.NewRecorder()
service := NewService(log)

service.Retry(ctx)

warnings := rec.FilterMessageContains("giving up")
require.Len(t, warnings, 1)
require.Equal(t, "billing", warnings[0].Attrs["component"])
require.ErrorIs(t, warnings[0].Attrs["error"].(error), context.DeadlineExceeded)

A Record carries the message, the level, the time and every attribute: With attributes included, groups flattened to dotted keys (piece.id), and errors kept as the error value rather than its message, so errors.Is still works on them. NewRecorderAt(level) drops what is below level, and Enabled reports it, so code that skips building an expensive line when the level is off is tested as it runs.

To see the lines and assert on them, combine the two:

recorded, rec := logtest.NewRecorder()
log := slog.New(slog.NewMultiHandler(logtest.New(t).Handler(), recorded.Handler()))