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On Windows, long-UNC paths are needed to inspect and modify paths with more than 260 characters. The os.path.split() function behaves incorrectly in the presence of a long-UNC prefix //?/ or \\?\. Consider iterating d = os.path.split(d)[0] with d = '//?/e:/python-test/dir'. Then the values of d are as follows:
'//?/e:/python-test/dir'
'//?/e:/python-test'
'//?/e:'
'//?'
'//'
The two last splits are the incorrect ones, where the splitting should end at '//?/e:'.
One consequence of this is that os.makedirs(d) crashes, because it attempts to run os.mkdir('//') at the bottom of its recursion. The same thing happens when replacing all / with \\ in the above.

By inspecting the code for os.path.split() in ntpath.py, one sees that the problem is actually in os.path.splitdrive(), which does not handle long-UNC paths.

It seems to me that this problem can be fixed by replacing splitdrive with splitunc at line 170 in ntpath.py.

splitunc is deprecated since 3.1. It is also written in the code that "Paths containing drive letters never have an UNC part"

This behavior reproduced only on 2.7. See issue5799 which changed the behavior for 3.1.

I propose to close this issue as "won't fix". A long-UNC prefix support is a new feature and can't be applied to 2.7. As workaround use splitunc() in Python prior to 3.1.

I agree. Larry H.'s patch in #5799 was called an enhancement and is explicitly a replacement for splitunc. The latter was deprecated and may disappear in 3.4.

ISTM that fixing this for 3.x (3? 4?) is worthwhile though. Or did somebody already fix it in 3.x?

Serhiy claims that the commit of Mark's version of your patch in #5799 fixed this in 3.1. Retesting in IDLE, 3.3, Win7:
>>> import os
>>> d = '//?/e:/python-test/dir'
>>> d = os.path.split(d)[0]
>>> d
'//?/e:/python-test'
>>> d = os.path.split(d)[0]
>>> d
'//?/e:/'
>>> d = os.path.split(d)[0]
>>> d
'//?/e:/'
So the splitting ends where Kalle agrees it should.