[Python-Dev] A wart which should have been repaired in 3.0? (original) (raw)

Paul Moore p.f.moore at gmail.com
Tue Dec 30 10:36:26 CET 2008

2008/12/30 Phillip J. Eby <pje at telecommunity.com>:

You know, all this path separator and list complication isn't really necessary, when you can just take the os.path.dirname() of the return from commonprefix().

Perhaps we could just add that recommendation to the docs?

Actually, consider the following (on Windows):

python Python 2.6.1 (r261:67517, Dec 4 2008, 16:51:00) [MSC v.1500 32 bit (Intel)] on win32 Type "help", "copyright", "credits" or "license" for more information.

import os os.path.commonprefix(["foo\bar\baz", "foo/bar/boink"]) 'foo'

This very clearly shows that commonprefix is a string operation rather than a path operation, as it does not respect the equivalence of os.sep and os.altsep. In path semantics, the common prefix is "foo/bar" (or equivalently "foo\bar").

I'm not sure how to deal with this, except by recommending that all paths passed to os.path.commonprefix should at the very least be normalised via os.path.normpath first - which starts to get clumsy fast. So the "recommended" usage to get the common directory is

paths = [...]
common = os.path.dirname(os.path.commonprefix([os.path.normpath(p)

for p in paths]))

Hmm...

Paul.

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