Add unit tests for easy-install.pth uninstall

[MHendricks]

Resolves issue #6516.

If uninstalling a editable package in Python 2.7 that was installed using a UNC file path, the path is not removed from the easy-install.pth file.

Change use of os.path.splitdrive to os.path.isabs and add unit tests for .pth file handling.

[pfmoore]

One minor comment, but it doesn't need addressing and I'm +1 on this change.

[pfmoore]

Technically, this will behave differently on a non-absolute path with a drive letter, like C:foo\bar. To be honest, though, I can't imagine why we'd ever encounter a real life case where that would happen, so I'm inclined to ignore the issue. And as a general point, isabs seems clearer to me.

My only (very slight) reservation is that not using isabs in the first place is non-obvious enough that thee may have been a reason for it. But it seems to have been introduced in 770930d, without comment, so I'm inclined to think there was no particular motive.

[pfmoore]

Looks like there are some test failures that will need addressing, though :-)

[MHendricks]

https://stackoverflow.com/a/42014874 The problem with the test was due to a change in python 2.7.8 where isabs returns False if you don't pass more than just the server\share. os.path.isabs('\\\\server\\share') returns False on 2.7.8+ while os.path.isabs('\\\\server\\share\\') returns True. I was developing on a older version of python and though I had tested on newer python installs, but apparently not.

Testing on 2.7.15, I get the following results:

>>> p = '\\\\example\\share\\case'; print not os.path.isabs(p), not os.path.splitdrive(p)[0], os.path.splitdrive(p)
False False ('\\\\example\\share', '\\case')
>>> p = '\\\\example\\share\\'; print not os.path.isabs(p), not os.path.splitdrive(p)[0], os.path.splitdrive(p)
False False ('\\\\example\\share', '\\')
>>> p = '\\\\example\\share'; print not os.path.isabs(p), not os.path.splitdrive(p)[0], os.path.splitdrive(p)
True False ('\\\\example\\share', '')
>>> p = '\\\\example'; print not os.path.isabs(p), not os.path.splitdrive(p)[0], os.path.splitdrive(p)
False True ('', '\\\\example')
>>> p = 'C:\\foo\\bar'; print not os.path.isabs(p), not os.path.splitdrive(p)[0], os.path.splitdrive(p)
False False ('C:', '\\foo\\bar')
>>> p = 'C:foo\\bar'; print not os.path.isabs(p), not os.path.splitdrive(p)[0], os.path.splitdrive(p)
True False ('C:', 'foo\\bar')
>>> p = 'foo\\bar'; print not os.path.isabs(p), not os.path.splitdrive(p)[0], os.path.splitdrive(p)
True True ('', 'foo\\bar')

It looks like isabs fails if you are using a unc share as the root of your pip package, which I think is unlikely, but would be supported if using the original splitdrive.

The root of the problem was due to me using a old un-supported version of python. If you want I can revert the change to isabs, but leave the unit test changes, or just close the pull request.

[cjerdonek]

If nothing else, I think it would be worth documenting your findings in a code comment so it won’t need to be rediscovered and because it seems obscure.

[pfmoore]

Excellent bit of diagnosis! I agree with @cjerdonek that documenting the reason that isabs won't work would be very useful. Personally, I'm not sure that the behaviour of isabs in this edge case makes much sense, but I can see why consistency across os.path APIs is a reasonable justification for picking this particular behaviour. And I'm +1 on keeping the tests, simply because more tests is good.

[MHendricks]

Documentation added to req_uninstall.py and original splitdrive check restored.

[BrownTruck]

Hello!

I am an automated bot and I have noticed that this pull request is not currently able to be merged. If you are able to either merge the master branch into this pull request or rebase this pull request against master then it will be eligible for code review and hopefully merging!

[chrahunt]

Anything else needed for this?

[pradyunsg]

I have some code style disagreements but I'm okay functionally.

I'll defer to others to merge though.

[pfmoore]

Thanks for the contribution!