Code:

{
	ts, _ := time.Parse(time.RFC3339, "2000-12-24T12:30:40Z")
	internal.Pid = 123 // To get consistent output for each run.
	config := textlogger.NewConfig(
		textlogger.FixedTime(ts), // To get consistent output for each run.
		textlogger.Verbosity(4),  // Matches Kubernetes "debug" level.
		textlogger.Output(os.Stdout),
	)
	logger := textlogger.NewLogger(config)

	logger.V(4).Info("A debug message")
	logger.V(5).Info("A debug message with even lower level, not printed.")
	logger.Info("An info message")
	logger.Error(errors.New("fake error"), "An error")
	logger.WithValues("int", 42).Info("With values",
		"duration", time.Second,
		"float", 3.12,
		"coordinates", coordinatesMarshaler{x: 100, y: 200},
		"variables", variables{A: 1, B: 2},
	)
	// The logr API supports skipping functions during stack unwinding, in contrast to slog.
	someHelper(logger, "hello world")

	// Output:
	// I1224 12:30:40.000000     123 textlogger_test.go:56] "A debug message"
	// I1224 12:30:40.000000     123 textlogger_test.go:58] "An info message"
	// E1224 12:30:40.000000     123 textlogger_test.go:59] "An error" err="fake error"
	// I1224 12:30:40.000000     123 textlogger_test.go:60] "With values" int=42 duration="1s" float=3.12 coordinates={"X":100,"Y":200} variables={"A":1,"B":2}
	// I1224 12:30:40.000000     123 textlogger_test.go:67] "hello world"
}

Output:

I1224 12:30:40.000000     123 textlogger_test.go:56] "A debug message"
I1224 12:30:40.000000     123 textlogger_test.go:58] "An info message"
E1224 12:30:40.000000     123 textlogger_test.go:59] "An error" err="fake error"
I1224 12:30:40.000000     123 textlogger_test.go:60] "With values" int=42 duration="1s" float=3.12 coordinates={"X":100,"Y":200} variables={"A":1,"B":2}
I1224 12:30:40.000000     123 textlogger_test.go:67] "hello world"

Code:play 

//go:build go1.21
// +build go1.21

/*
Copyright 2023 The Kubernetes Authors.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

package main

import (
	"errors"
	"log/slog"
	"os"
	"time"

	"github.com/go-logr/logr/slogr"
	internal "k8s.io/klog/v2/internal/buffer"
	"k8s.io/klog/v2/textlogger"
)

var _ slog.LogValuer = coordinates{}

type coordinates struct {
	x, y int
}

func (c coordinates) LogValue() slog.Value {
	return slog.GroupValue(slog.Attr{Key: "X", Value: slog.IntValue(c.x)}, slog.Attr{Key: "Y", Value: slog.IntValue(c.y)})
}

func main() {
	ts, _ := time.Parse(time.RFC3339, "2000-12-24T12:30:40Z")
	internal.Pid = 123 // To get consistent output for each run.
	config := textlogger.NewConfig(
		textlogger.FixedTime(ts), // To get consistent output for each run.
		textlogger.Verbosity(4),  // Matches slog.LevelDebug.
		textlogger.Output(os.Stdout),
	)
	logrLogger := textlogger.NewLogger(config)
	slogHandler := slogr.NewSlogHandler(logrLogger)
	slogLogger := slog.New(slogHandler)

	slogLogger.Debug("A debug message")
	slogLogger.Log(nil, slog.LevelDebug-1, "A debug message with even lower level, not printed.")
	slogLogger.Info("An info message")
	slogLogger.Warn("A warning")
	slogLogger.Error("An error", "err", errors.New("fake error"))

	// The slog API supports grouping, in contrast to the logr API.
	slogLogger.WithGroup("top").With("int", 42, slog.Group("variables", "a", 1, "b", 2)).Info("Grouping",
		"sub", slog.GroupValue(
			slog.Attr{Key: "str", Value: slog.StringValue("abc")},
			slog.Attr{Key: "bool", Value: slog.BoolValue(true)},
			slog.Attr{Key: "bottom", Value: slog.GroupValue(slog.Attr{Key: "coordinates", Value: slog.AnyValue(coordinates{x: -1, y: -2})})},
		),
		"duration", slog.DurationValue(time.Second),
		slog.Float64("pi", 3.12),
		"e", 2.71,
		"moreCoordinates", coordinates{x: 100, y: 200},
	)

	// slog special values are also supported when passed through the logr API.
	// This works with the textlogger, but might not work with other implementations
	// and thus isn't portable. Passing attributes (= key and value in a single
	// parameter) is not supported.
	logrLogger.Info("slog values",
		"variables", slog.GroupValue(slog.Int("a", 1), slog.Int("b", 2)),
		"duration", slog.DurationValue(time.Second),
		"coordinates", coordinates{x: 100, y: 200},
	)

}

Output:

I1224 12:30:40.000000     123 textlogger_slog_test.go:55] "A debug message"
I1224 12:30:40.000000     123 textlogger_slog_test.go:57] "An info message"
W1224 12:30:40.000000     123 textlogger_slog_test.go:58] "A warning"
E1224 12:30:40.000000     123 textlogger_slog_test.go:59] "An error" err="fake error"
I1224 12:30:40.000000     123 textlogger_slog_test.go:62] "Grouping" top.int=42 top.variables={"a":1,"b":2} top.sub={"str":"abc","bool":true,"bottom":{"coordinates":{"X":-1,"Y":-2}}} top.duration="1s" top.pi=3.12 top.e=2.71 top.moreCoordinates={"X":100,"Y":200}
I1224 12:30:40.000000     123 textlogger_slog_test.go:78] "slog values" variables={"a":1,"b":2} duration="1s" coordinates={"X":100,"Y":200}

Code:play 

package main

import (
	"bytes"
	"fmt"
	"regexp"

	"k8s.io/klog/v2/textlogger"
)

var headerRe = regexp.MustCompile(`([IE])[[:digit:]]{4} [[:digit:]]{2}:[[:digit:]]{2}:[[:digit:]]{2}\.[[:digit:]]{6}[[:space:]]+[[:digit:]]+ example_test.go:[[:digit:]]+\] `)

func main() {
	var buffer bytes.Buffer
	config := textlogger.NewConfig(textlogger.Verbosity(1), textlogger.Output(&buffer))
	logger := textlogger.NewLogger(config)

	logger.Info("initial verbosity", "v", config.Verbosity().String())
	logger.V(2).Info("now you don't see me")
	if err := config.Verbosity().Set("2"); err != nil {
		logger.Error(err, "setting verbosity to 2")
	}
	logger.V(2).Info("now you see me")
	if err := config.Verbosity().Set("1"); err != nil {
		logger.Error(err, "setting verbosity to 1")
	}
	logger.V(2).Info("now I'm gone again")

	fmt.Print(headerRe.ReplaceAllString(buffer.String(), "${1}...] "))

}

Output:

I...] "initial verbosity" v="1"
I...] "now you see me"

Code:play 

package main

import (
	"os"
	"time"

	internal "k8s.io/klog/v2/internal/buffer"
	"k8s.io/klog/v2/textlogger"
)

func main() {
	ts, _ := time.Parse(time.RFC3339, "2000-12-24T12:30:40Z")
	internal.Pid = 123 // To get consistent output for each run.
	backtraceCounter := 0
	config := textlogger.NewConfig(
		textlogger.FixedTime(ts), // To get consistent output for each run.
		textlogger.Backtrace(func(_ /* skip */ int) (filename string, line int) {
			backtraceCounter++
			if backtraceCounter == 1 {
				// Simulate "missing information".
				return "", 0
			}
			return "fake.go", 42

			// A real implementation could use Ginkgo:
			//
			// import ginkgotypes "github.com/onsi/ginkgo/v2/types"
			//
			// location := ginkgotypes.NewCodeLocation(skip + 1)
			// return location.FileName, location.LineNumber
		}),
		textlogger.Output(os.Stdout),
	)
	logger := textlogger.NewLogger(config)

	logger.Info("First message")
	logger.Info("Second message")

}

Output:

I1224 12:30:40.000000     123 ???:1] "First message"
I1224 12:30:40.000000     123 fake.go:42] "Second message"

Code:play 

package main

import (
	"bytes"
	"errors"
	"fmt"

	"k8s.io/klog/v2/textlogger"
)

func main() {
	var buffer bytes.Buffer
	config := textlogger.NewConfig(
		textlogger.WithHeader(false),
		textlogger.Output(&buffer),
	)
	logger := textlogger.NewLogger(config)

	logger.Error(errors.New("fake error"), "Something broke", "id", 42)
	logger.WithName("name").WithValues("key", "value").WithCallDepth(0).Info("Still no header")
	fmt.Println(buffer.String())

}

Output:

"Something broke" err="fake error" id=42
"Still no header" logger="name" key="value"