How to enable line numbers for log, but only for errors?

[mcandre (Andrew)]

I see that log.SetFlags(log.LstdFlags | log.Lshortfile) enables line numbers for all log output. But is there a way to enable line numbers just for error objects, Fatal, and Fatalf?

[inancgumus]

There is not any way to enable line numbers just for Fatal and Fatalf funcs. However, you can use struct embedding to customize it like so:

package main

import (
	"io"
	"log"
	"os"
)

type MyLog struct {
	*log.Logger
	fatal *log.Logger
}

func New(out io.Writer, prefix string, flag int) *MyLog {
	l := &MyLog{
		Logger: log.New(out, prefix, flag),
		fatal:  log.New(out, prefix, flag|log.LstdFlags|log.Lshortfile),
	}
	return l
}

func (l *MyLog) Fatal(v ...interface{}) {
	l.fatal.Fatal(v...)
}

func main() {
	logger := New(os.Stderr, "logger: ", log.Lshortfile)

	logger.Print("Hello, log file!")
	logger.Fatal("Hello, log file!")
}

https://play.golang.org/p/9lr3FL5sYeN