Raise error if --user and --target arguments are used together

[sinscary]

closes #7249

This PR adds a validation which runs while parsing the command and throws a CommandError if --user and --target args are used together in same command.

[sinscary]

@pradyunsg please suggest a good error message. I am really bad with this.

[sinscary]

@pradyunsg can you review the changes?

[uranusjr]

I would handle this in InstallCommand instead, since it is the only context where --user and --target are mutually exclusive.

It is much easier to handle it in run() as well; optparse documentation conveniently provides an example; you can simply write something like

def run(self, options, args):
    if options.use_user_site and options.target_dir is not None:
        raise CommandError('--user and --target cannot be used together.')
    # ...

[sinscary]

@uranusjr My bad. I didn't realise --target was being used with install command only. But don't you think error out early would be better than executing all the code to populate the options, import install module etc. and then error it out. I mean we already know these both args are not allowed together.

[uranusjr]

I mean we already know these both args are not allowed together.

Indeed, but you’re already missing -t which is an alias of --target. And what happens if we add more aliases to either argument? Populating the whole options is actually a good thing IMO since it provides both consistency and safety.

[sinscary]

Oh yeah that makes sense, Looks like I didn't consider all the cases 😞. Well, will make the changes. Thanks for the review 😄

[sinscary]

As we are on the topic. Can you suggest a better error message to show?

[uranusjr]

That I have absolutely no suggestions. I think the current message is good enough, but I’m terrible at this 😆

[sinscary]

@uranusjr @pradyunsg Can you review the changes again?

[uranusjr]

Maybe specify in pip install?

[uranusjr]

One nitpicking, otherwise LGTM 👍

[sinscary]

@uranusjr done. But Looks like this test test_distutils_config_file_read is failing for python2 for everyone.

[sinscary]

@uranusjr @pradyunsg meanwhile tests run, just curious to know why does pip have so many checks 😕 ? 45 checks, I mean that's a lot compared to other pypa projects.

[pradyunsg]

@sinscary https://pip.pypa.io/en/stable/development/ci/