src/log/slog/example_custom_levels_test.go

// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package slog_test

import (
	"context"
	"log/slog"
	"os"
)

// This example demonstrates using custom log levels and custom log level names.
// In addition to the default log levels, it introduces Trace, Notice, and
// Emergency levels. The ReplaceAttr changes the way levels are printed for both
// the standard log levels and the custom log levels.
func ExampleHandlerOptions_customLevels() {
	// Exported constants from a custom logging package.
	const (
		LevelTrace     = slog.Level(-8)
		LevelDebug     = slog.LevelDebug
		LevelInfo      = slog.LevelInfo
		LevelNotice    = slog.Level(2)
		LevelWarning   = slog.LevelWarn
		LevelError     = slog.LevelError
		LevelEmergency = slog.Level(12)
	)

	th := slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
		// Set a custom level to show all log output. The default value is
		// LevelInfo, which would drop Debug and Trace logs.
		Level: LevelTrace,

		ReplaceAttr: func(groups []string, a slog.Attr) slog.Attr {
			// Remove time from the output for predictable test output.
			if a.Key == slog.TimeKey {
				return slog.Attr{}
			}

			// Customize the name of the level key and the output string, including
			// custom level values.
			if a.Key == slog.LevelKey {
				// Rename the level key from "level" to "sev".
				a.Key = "sev"

				// Handle custom level values.
				level := a.Value.Any().(slog.Level)

				// This could also look up the name from a map or other structure, but
				// this demonstrates using a switch statement to rename levels. For
				// maximum performance, the string values should be constants, but this
				// example uses the raw strings for readability.
				switch {
				case level < LevelDebug:
					a.Value = slog.StringValue("TRACE")
				case level < LevelInfo:
					a.Value = slog.StringValue("DEBUG")
				case level < LevelNotice:
					a.Value = slog.StringValue("INFO")
				case level < LevelWarning:
					a.Value = slog.StringValue("NOTICE")
				case level < LevelError:
					a.Value = slog.StringValue("WARNING")
				case level < LevelEmergency:
					a.Value = slog.StringValue("ERROR")
				default:
					a.Value = slog.StringValue("EMERGENCY")
				}
			}

			return a
		},
	})

	logger := slog.New(th)
	ctx := context.Background()
	logger.Log(ctx, LevelEmergency, "missing pilots")
	logger.Error("failed to start engines", "err", "missing fuel")
	logger.Warn("falling back to default value")
	logger.Log(ctx, LevelNotice, "all systems are running")
	logger.Info("initiating launch")
	logger.Debug("starting background job")
	logger.Log(ctx, LevelTrace, "button clicked")

	// Output:
	// sev=EMERGENCY msg="missing pilots"
	// sev=ERROR msg="failed to start engines" err="missing fuel"
	// sev=WARNING msg="falling back to default value"
	// sev=NOTICE msg="all systems are running"
	// sev=INFO msg="initiating launch"
	// sev=DEBUG msg="starting background job"
	// sev=TRACE msg="button clicked"
}