Copying flags

[jzelinskie]

There are use cases for testing or re-using flags from other programs where you actually want to copy flags from a flagset into a new one and not simply add it. There are lots of private fields that make this difficult/impossible with the existing API.

[tomasaschan]

This issue has not been updated in a long time. Is it still relevant?

(If I receive no answer before the end of the summer, I'll close it. Feel free to re-open if you come here after that happens and it is relevant to you - but if you do, please also comment and specify what outcome you're interested in!)

[jzelinskie]

It is still relevant.

Our use case is creating two applications that share the same flags. We're lucky enough to be in control of both applications, but if we were not I believe you'd have to copy and paste code and keep it in sync with the upstream. We ultimately turned our flag registration into a reusable library, but it adds unnecessary complexity to the upstream.

[tomasaschan]

Are those applications open source, so I can take a look at what you do today? If not, is it possible to provide some minimal example of what you are doing today and what you would like to be able to do instead?

It seems to me like a better option for your use case might be to have a function to either register flags on a provided flag set, or to return a new one with flags registered, but that sounds like it might be what you're already doing, so I'd like to understand why that is not satisfactory.

[brandon1024]

Although not a perfect solution, you could simply iterate over the FlagSet from application A and registering flags with FlagSet.Var() of application B.

In pseudocode:

a.VisitAll(func(f *pflag.Flag) {
    b.Var(f.Value, f.Name, f.Usage)
})

This approach has caveats though -- there's some flag state that won't be copied over (like Hidden, Annotations, etc.), so you would need to handle that as well, but it could be a reasonable approach.