github.com/sirupsen/logrus v1.6.0: Formatter

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify logrus.Formatter in pkg:golang/github.com/sirupsen/logrus@v1.6.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/github.com/sirupsen/logrus@v1.6.0
Demonstrate these symbols/APIs:
  - logrus.Formatter

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:2e78ee45eabb1e65181ea52d2914b2043a7425f4cc3e8cfbe1cc75e6abb8c0f6","contract":["logrus.Formatter formats a logrus.Entry into bytes according to custom formatting logic","logrus.SetFormatter applies a custom Formatter to the Logger so output reflects the custom format","logrus.Formatter receives fields injected via logrus.WithFields in entry.Data"],"goal":"verify logrus.Formatter in pkg:golang/github.com/sirupsen/logrus@v1.6.0","kind":"HOW","packages":["pkg:golang/github.com/sirupsen/logrus@v1.6.0"],"schemaVersion":1,"symbols":["logrus.Formatter"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/github.com/sirupsen/logrus@v1.6.0"],"schemaVersion":1,"subject":"pkg:golang/github.com/sirupsen/logrus@v1.6.0","symbols":["logrus.Formatter"],"verifierAdapter":"golang@1"}

module example.com/logrus-formatter-sample

go 1.26.6

require github.com/sirupsen/logrus v1.6.0

require (
	github.com/konsorten/go-windows-terminal-sequences v1.0.3 // indirect
	golang.org/x/sys v0.0.0-20190422165155-953cdadca894 // indirect
)

github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
github.com/konsorten/go-windows-terminal-sequences v1.0.3 h1:CE8S1cTafDpPvMhIxNJKvHsGVBgn1xWYf1NbHQhywc8=
github.com/konsorten/go-windows-terminal-sequences v1.0.3/go.mod h1:T0+1ngSBFLxvqU3pZ+m/2kptfBszLMUkC4ZK/EgS/cQ=
github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
github.com/sirupsen/logrus v1.6.0 h1:UBcNElsrwanuuMsnGSlYmtmgbb23qDR5dG+6X6Oo89I=
github.com/sirupsen/logrus v1.6.0/go.mod h1:7uNnSEd1DgxDLC74fIahvMZmmYsHGZGEOFrfsX/uA88=
github.com/stretchr/testify v1.2.2 h1:bSDNvY7ZPG5RlJ8otE/7V6gMiyenm9RtJ7IUVIAoJ1w=
github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
golang.org/x/sys v0.0.0-20190422165155-953cdadca894 h1:Cz4ceDQGXuKRnVBDTS23GTn/pU5OE2C0WrNTOYK1Uuc=
golang.org/x/sys v0.0.0-20190422165155-953cdadca894/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=

package main

import (
	"bytes"
	"fmt"
	"io"
	"strings"

	"github.com/sirupsen/logrus"
)

// CustomPrefixFormatter implements logrus.Formatter by prefixing log lines.
type CustomPrefixFormatter struct {
	Prefix string
}

// Format formats a logrus.Entry into formatted bytes.
func (f *CustomPrefixFormatter) Format(entry *logrus.Entry) ([]byte, error) {
	var b bytes.Buffer
	b.WriteString(fmt.Sprintf("[%s] %s: %s", f.Prefix, strings.ToUpper(entry.Level.String()), entry.Message))
	for k, v := range entry.Data {
		b.WriteString(fmt.Sprintf(" %s=%v", k, v))
	}
	b.WriteByte('\n')
	return b.Bytes(), nil
}

func main() {
	logrus.SetOutput(io.Discard)
	formatter := &CustomPrefixFormatter{Prefix: "APP"}
	logrus.SetFormatter(formatter)

	logrus.WithField("component", "worker").Info("service started")
	fmt.Println("Custom formatter configured and sample log emitted")
}

{
  "schemaVersion": 1,
  "goal": "verify logrus.Formatter in pkg:golang/github.com/sirupsen/logrus@v1.6.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/github.com/sirupsen/logrus@v1.6.0"
  ],
  "symbols": [
    "logrus.Formatter"
  ]
}

package main

import (
	"bytes"
	"fmt"
	"os"
	"strings"

	"github.com/sirupsen/logrus"
)

// CustomPrefixFormatter implements logrus.Formatter.
type CustomPrefixFormatter struct {
	Prefix string
}

// Format renders a logrus.Entry into formatted bytes.
func (f *CustomPrefixFormatter) Format(entry *logrus.Entry) ([]byte, error) {
	var b bytes.Buffer
	b.WriteString(fmt.Sprintf("[%s] %s: %s", f.Prefix, strings.ToUpper(entry.Level.String()), entry.Message))
	for k, v := range entry.Data {
		b.WriteString(fmt.Sprintf(" %s=%v", k, v))
	}
	b.WriteByte('\n')
	return b.Bytes(), nil
}

func main() {
	// Assertion 1: logrus.Formatter formats a logrus.Entry into bytes according to custom formatting logic
	formatter := &CustomPrefixFormatter{Prefix: "TEST"}
	var ifaceCheck logrus.Formatter = formatter
	_ = ifaceCheck

	logger := logrus.New()
	entry := logrus.NewEntry(logger)
	entry.Level = logrus.InfoLevel
	entry.Message = "hello world"
	entry.Data = logrus.Fields{"user": "alice"}

	formattedBytes, err := formatter.Format(entry)
	if err != nil {
		fmt.Fprintf(os.Stderr, "assertion 1 failed: unexpected format error: %v\n", err)
		os.Exit(1)
	}
	formattedStr := string(formattedBytes)
	if !strings.HasPrefix(formattedStr, "[TEST] INFO: hello world") {
		fmt.Fprintf(os.Stderr, "assertion 1 failed: formatted prefix mismatch, got: %q\n", formattedStr)
		os.Exit(1)
	}
	if !strings.Contains(formattedStr, "user=alice") {
		fmt.Fprintf(os.Stderr, "assertion 1 failed: missing user field in formatted output: %q\n", formattedStr)
		os.Exit(1)
	}

	// Assertion 2: logrus.SetFormatter applies a custom Formatter to the Logger so output reflects the custom format
	var outBuf bytes.Buffer
	testLogger := logrus.New()
	testLogger.SetOutput(&outBuf)
	testLogger.SetFormatter(&CustomPrefixFormatter{Prefix: "LOGGER_TEST"})

	testLogger.Info("message through logger")
	output := outBuf.String()
	expectedPrefix := "[LOGGER_TEST] INFO: message through logger\n"
	if output != expectedPrefix {
		fmt.Fprintf(os.Stderr, "assertion 2 failed: expected %q, got %q\n", expectedPrefix, output)
		os.Exit(1)
	}

	// Assertion 3: logrus.Formatter receives fields injected via logrus.WithFields in entry.Data
	outBuf.Reset()
	testLogger.WithFields(logrus.Fields{
		"request_id": "req-1234",
		"status":     200,
	}).Warn("request completed")

	warnOutput := outBuf.String()
	if !strings.HasPrefix(warnOutput, "[LOGGER_TEST] WARNING: request completed") {
		fmt.Fprintf(os.Stderr, "assertion 3 failed: prefix mismatch in warn output: %q\n", warnOutput)
		os.Exit(1)
	}
	if !strings.Contains(warnOutput, "request_id=req-1234") || !strings.Contains(warnOutput, "status=200") {
		fmt.Fprintf(os.Stderr, "assertion 3 failed: missing fields in warn output: %q\n", warnOutput)
		os.Exit(1)
	}

	fmt.Println("PASS: logrus.Formatter contract verified")
}