blob: 75b66b716f3a0bbf355677e1445b4167e9d3dfaf [file] [log] [blame]
// Copyright 2022 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
import (
// TextHandler is a Handler that writes Records to an io.Writer as a
// sequence of key=value pairs separated by spaces and followed by a newline.
type TextHandler struct {
// NewTextHandler creates a TextHandler that writes to w,
// using the given options.
// If opts is nil, the default options are used.
func NewTextHandler(w io.Writer, opts *HandlerOptions) *TextHandler {
if opts == nil {
opts = &HandlerOptions{}
return &TextHandler{
json: false,
w: w,
opts: *opts,
// Enabled reports whether the handler handles records at the given level.
// The handler ignores records whose level is lower.
func (h *TextHandler) Enabled(_ context.Context, level Level) bool {
return h.commonHandler.enabled(level)
// WithAttrs returns a new TextHandler whose attributes consists
// of h's attributes followed by attrs.
func (h *TextHandler) WithAttrs(attrs []Attr) Handler {
return &TextHandler{commonHandler: h.commonHandler.withAttrs(attrs)}
func (h *TextHandler) WithGroup(name string) Handler {
return &TextHandler{commonHandler: h.commonHandler.withGroup(name)}
// Handle formats its argument Record as a single line of space-separated
// key=value items.
// If the Record's time is zero, the time is omitted.
// Otherwise, the key is "time"
// and the value is output in RFC3339 format with millisecond precision.
// If the Record's level is zero, the level is omitted.
// Otherwise, the key is "level"
// and the value of [Level.String] is output.
// If the AddSource option is set and source information is available,
// the key is "source" and the value is output as FILE:LINE.
// The message's key is "msg".
// To modify these or other attributes, or remove them from the output, use
// [HandlerOptions.ReplaceAttr].
// If a value implements [encoding.TextMarshaler], the result of MarshalText is
// written. Otherwise, the result of fmt.Sprint is written.
// Keys and values are quoted with [strconv.Quote] if they contain Unicode space
// characters, non-printing characters, '"' or '='.
// Keys inside groups consist of components (keys or group names) separated by
// dots. No further escaping is performed.
// Thus there is no way to determine from the key "a.b.c" whether there
// are two groups "a" and "b" and a key "c", or a single group "a.b" and a key "c",
// or single group "a" and a key "b.c".
// If it is necessary to reconstruct the group structure of a key
// even in the presence of dots inside components, use
// [HandlerOptions.ReplaceAttr] to encode that information in the key.
// Each call to Handle results in a single serialized call to
// io.Writer.Write.
func (h *TextHandler) Handle(_ context.Context, r Record) error {
return h.commonHandler.handle(r)
func appendTextValue(s *handleState, v Value) error {
switch v.Kind() {
case KindString:
case KindTime:
case KindAny:
if tm, ok := v.any.(encoding.TextMarshaler); ok {
data, err := tm.MarshalText()
if err != nil {
return err
// TODO: avoid the conversion to string.
return nil
if bs, ok := byteSlice(v.any); ok {
// As of Go 1.19, this only allocates for strings longer than 32 bytes.
return nil
s.appendString(fmt.Sprintf("%+v", v.Any()))
*s.buf = v.append(*s.buf)
return nil
// byteSlice returns its argument as a []byte if the argument's
// underlying type is []byte, along with a second return value of true.
// Otherwise it returns nil, false.
func byteSlice(a any) ([]byte, bool) {
if bs, ok := a.([]byte); ok {
return bs, true
// Like Printf's %s, we allow both the slice type and the byte element type to be named.
t := reflect.TypeOf(a)
if t != nil && t.Kind() == reflect.Slice && t.Elem().Kind() == reflect.Uint8 {
return reflect.ValueOf(a).Bytes(), true
return nil, false
func needsQuoting(s string) bool {
if len(s) == 0 {
return true
for i := 0; i < len(s); {
b := s[i]
if b < utf8.RuneSelf {
// Quote anything except a backslash that would need quoting in a
// JSON string, as well as space and '='
if b != '\\' && (b == ' ' || b == '=' || !safeSet[b]) {
return true
r, size := utf8.DecodeRuneInString(s[i:])
if r == utf8.RuneError || unicode.IsSpace(r) || !unicode.IsPrint(r) {
return true
i += size
return false