Golang: fmt, variadic args and %!(EXTRA type=value) error


I’m implementing a wrapper around the standard log package to make a logger with various log levels.

I have the following interface:

type Logger interface {
  Trace(fmt string, args ...interface{})
  Debug(fmt string, args ...interface{})
  Info(fmt string, args ...interface{})
  Warn(fmt string, args ...interface{})
  Error(fmt string, args ...interface{})
  Fatal(fmt string, args ...interface{})
  Panic(fmt string, args ...interface{})

In the implementation I have something like this (not the exact code)

func Info(format string, args ...interface{}){
  msg := fmt.Sprintf(format, args...)

Now, assume I call my library like this:

logger.Info("Hello %s", "World")

I get the printout: “Hello %!(EXTRA string=WORLD)”, instead of the expected “Hello World”. There a similar output if I do

msg := fmt.Sprintf(format, args)

This returns “Hello World%!EXTRA []interface{}=[]”.


I can’t reproduce this behavior. Are you sure it’s not a simple error that you forgot to show here?


package main

import "fmt"

func Info(format string, args ...interface{}){
    msg := fmt.Sprintf(format, args...)

func main() {
    Info("Hello %s", "World")


Hello World

According to the fmt docs, %!(EXTRA string=WORLD) is added to the string when you pass extra parameters, unexpected by the format. Maybe you are using the format string "Hello World" instead of "Hello %s", or passing the argument twice?

Answered By – BoppreH

Answer Checked By – Marie Seifert (GoLangFix Admin)

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