flag: antipattern - failf error handler prints usage unconditionally

[abitrolly]

Go version

1.24

Output of go env in your module/workspace:

No.

What did you do?

Minimal flags example (https://go.dev/play/p/r6dDUoerjTI):

package main

import "flag"

func main() {
        flag.Parse()
}

Running this example with invalid flag always prints usage that is impossible to turn off regardless of error handling strategy https://pkg.go.dev/flag#ErrorHandling

What did you see happen?

$ go run minimal.go -ggg
flag provided but not defined: -ggg
Usage of /home/anatoli/.cache/go-build/8d/8dae0dadd55b6ae9e6d4bca4d6c85e20970b537337a98a5006eaeef926f92b5f-d/minimal:
exit status 2

The problem is in failf function https://cs.opensource.google/go/go/+/refs/tags/go1.24.2:src/flag/flag.go;l=1060 that outputs the usage two levels below Parse where it should be handled.

Parse --> parseOne --> failf --> usage

The similar report #63977

What did you expect to see?

At least some way to suppress usage message in errors.

Right now the only way to do this is to completely turn off all flag output and handle it all manually.

[gabyhelp]

Related Issues

• proposal: flag: simplify `failf` output in `FlagSet` #63977
• flag: ignores failures when the flag is not parseable into a boolean #3869 (closed)
• flag: global Usage func disregards FlagSet.SetOutput #20998 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[seankhliao]

This is the documented and intended behaviour of FlagSet.Usage:

https://pkg.go.dev/flag#FlagSet

Usage is the function called when an error occurs while parsing flags.
...
What happens after Usage is called depends on the ErrorHandling setting; for the command line, this defaults to ExitOnError, which exits the program after calling Usage.

ErrorHandling only affects what happens after Usage.
Closing as working as intended.

[abitrolly]

@seankhliao but do you agree on the very strong code smell here?

[gopherbot]

Change https://go.dev/cl/664755 mentions this issue: flag: move usage printing from error factory to Parse