When using a flagset, errors are printed to stderr in addition to being returned by flag.Parse(). It is redundant, since the caller can print the error if desired. It also creates a small issue in a situation where multiple flagsets are used to parse a single command line.

I have a command line interface that provides a wrapper to multiple modules. The command line has its own flagset [1], and forward the flags it cannot parse to one of many packages where specific flagsets are defined [2, 3]. The issue is that the first call to Parse() detects the flags it doesn't know and prints an error to stderr directly. I would like to avoid printing this error. I ended up setting the output to os.DevNull around the call to Parse() to prevent displaying it to the user, but it feels like a hack [4].

The culpric is here: https://github.com/golang/go/blob/master/src/flag/flag.go#L793
I suggest removing that line and only returning the error to the caller through the Error value.

I'm not aware of a different way to split a flagset across packages, but if there's one, I'm very interested :)

[1] https://github.com/mozilla/mig/blob/master/src/mig/client/cmd/main.go#L71-L77
[2] https://github.com/mozilla/mig/blob/master/src/mig/modules/file/paramscreator.go#L285-L305
[3] https://github.com/mozilla/mig/blob/master/src/mig/modules/netstat/paramscreator.go#L171-L178
[4] https://github.com/mozilla/mig/blob/master/src/mig/client/cmd/main.go#L124