os.IsExist(err) vs os.IsNotExist(err)

isexist_vs_isnotexist.go

/*

  Watch out, os.IsExist(err) != !os.IsNotExist(err)

  They are error checkers, so use them only when err != nil, and you want to handle
  specific errors in a different way!
  
  Their main purpose is to wrap around OS error messages for you, so you don't have to test
  for Windows/Unix/Mobile/other OS error messages for "file exists/directory exists" and
  "file does not exist/directory does not exist"

  This way you can handle such an error, while still failing on other unexpected errors like "permission denied",
  "filesystem error", "wrong filename" error, etc.

*/

// We want to create a symlink, but the target path exists already :

if _, err := os.Symlink("/old/path", "/path/to/whatever"); os.IsExist(err) {
  // error happened, can't Symlink
  // can't create symlink because /path/to/whatever already exists
}

// We want to stat a file, but it doesn't exist :

if _, err := os.Stat("/path/to/whatever"); os.IsNotExist(err) {
  // error happened, can't Stat!
  // /path/to/whatever does not exist
}

// BUT :

// Anti-pattern : We want to stat a file, and continue if it exists :

if _, err := os.Stat("/file/that/exists"); os.IsExist(err) {
  // will never trigger!

  // why? because os.Stat runs normally if file exists.
  // it's expected behaviour for os.Stat, so it doesn't throw an error

  // os.IsExist() does not receive an error ( it's nil ), so it can't tell you
  // if the error message was "file not found"
}

// Instead :
// We want to stat a file, and continue if it exists :

_, err := os.Stat("/file/that/exists");

if err != nil {
	if os.IsNotExist(err) {
		// file does not exist, do something
	} else {
		// more serious errors
	}
}

// file exists.. continue with code here, or in else statement, or specify if err == nil { // do something }


/*
  os.IsExist(err) is good for cases when you expect the file to not exist yet,
  but the file actually exists :
*/

os.Symlink("/path/that/exists", "/path/to/symlink/target")
// os.IsExist(err) will trigger when target exists already

os.Mkdir(target)
// os.IsExist(err) will trigger because target path already exists

os.OpenFile(target, os.O_RDWR|os.O_CREATE|os.O_EXCL, 0600)
// os.IsExist(err) will trigger when target exists because O_EXCL means that file should not exist yet

/*
  os.IsNotExist(err) is good for more common cases where you expect the file to exists,
  but it actually doesn't exist :
*/

os.Chdir()
os.Stat()
os.Open()
os.OpenFile() // without os.O_EXCL
os.Chmod()
os.Chown()
os.Close()
os.Read()
os.ReadAt()
os.ReadDir()
os.Readdirnames()
os.Seek()
os.Truncate()
os.Write()
os.WriteAt()
os.WriteString()
// etc...

// More reading :

// http://stackoverflow.com/a/12518877
// http://stackoverflow.com/questions/25939584/file-both-exists-and-not-exists-in-go/25939743#25939743
// 
// https://github.com/golang/go/search?utf8=%E2%9C%93&q=IsExist
// https://golang.org/pkg/os/#IsExist
//
// http://stackoverflow.com/search?q=os.IsExist
// http://stackoverflow.com/search?q=os.IsNotExist