package standard library
go1.22.3 Latest Latest

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Published: May 7, 2024 License: BSD-3-Clause Imports: 8 Imported by: 4



Package slogtest implements support for testing implementations of log/slog.Handler.

Example (Parsing)

This example demonstrates one technique for testing a handler with this package. The handler is given a bytes.Buffer to write to, and each line of the resulting output is parsed. For JSON output, encoding/json.Unmarshal produces a result in the desired format when given a pointer to a map[string]any.

package main

import (

func main() {
	var buf bytes.Buffer
	h := slog.NewJSONHandler(&buf, nil)

	results := func() []map[string]any {
		var ms []map[string]any
		for _, line := range bytes.Split(buf.Bytes(), []byte{'\n'}) {
			if len(line) == 0 {
			var m map[string]any
			if err := json.Unmarshal(line, &m); err != nil {
				panic(err) // In a real test, use t.Fatal.
			ms = append(ms, m)
		return ms
	err := slogtest.TestHandler(h, results)
	if err != nil {





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func Run added in go1.22.0

func Run(t *testing.T, newHandler func(*testing.T) slog.Handler, result func(*testing.T) map[string]any)

Run exercises a slog.Handler on the same test cases as TestHandler, but runs each case in a subtest. For each test case, it first calls newHandler to get an instance of the handler under test, then runs the test case, then calls result to get the result. If the test case fails, it calls t.Error.

func TestHandler

func TestHandler(h slog.Handler, results func() []map[string]any) error

TestHandler tests a slog.Handler. If TestHandler finds any misbehaviors, it returns an error for each, combined into a single error with errors.Join.

TestHandler installs the given Handler in a slog.Logger and makes several calls to the Logger's output methods. The Handler should be enabled for levels Info and above.

The results function is invoked after all such calls. It should return a slice of map[string]any, one for each call to a Logger output method. The keys and values of the map should correspond to the keys and values of the Handler's output. Each group in the output should be represented as its own nested map[string]any. The standard keys slog.TimeKey, slog.LevelKey and slog.MessageKey should be used.

If the Handler outputs JSON, then calling encoding/json.Unmarshal with a `map[string]any` will create the right data structure.

If a Handler intentionally drops an attribute that is checked by a test, then the results function should check for its absence and add it to the map it returns.


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