[Python-Dev] path joining on Windows and imp.cache_from_source()

Brett Cannon brett at python.org
Sun Apr 22 08:00:11 CEST 2012

On Sun, Apr 22, 2012 at 01:44, Glenn Linderman <v+python at g.nevcal.com>wrote:

>  On 4/21/2012 8:53 PM, Brett Cannon wrote:
> imp.cache_from_source() (and thus also imp.source_from_cache()) has
> special semantics compared to how os.path.join() works. For instance, if
> you look at test_imp you will notice it tries to use the same path
> separator as is the farthest right in the path it is given::
>    self.assertEqual(imp.cache_from_source('\\foo\\bar/baz/qux.py',
> True), '\\foo\\bar\\baz/__pycache__/qux.{}.pyc'.format(self.tag))
>  But if you do the same basic operation using ntpath, you will notice it
> simply doesn't care::
>    >>> ntpath.join(ntpath.split('a\\b/c/d.py')[0], '__pycache__',
> 'd.cpython-32.pyc')
>   'a\\b/c\\__pycache__\\d.cpython-32.pyc
>  Basically imp.cache_from_source() goes to a bunch of effort to reuse the
> farthest right separator when there is an alternative separator *before*
> and path splitting is done. But if you look at ntpath.join(), it doesn't
> even attempt that much effort.
>  Now that we can reuse os.path.join() (directly for source_from_cache(),
> indirectly through easy algorithmic copying in cache_from_source()) do we
> want to keep the "special" semantics, or can I change it to match what
> ntpath would do when there can be more than one path separator on an OS
> (i.e. not do anything special)?
> Is there an issue here with importing from zip files, which use /
> separator, versus importing from the file system, which on Windows can use
> either / or \ ?  I don't know if imp.cache_from_source cares or is aware,
> but it is the only thing I can think of that might have an impact on such
> semantics.  (Well, the other is command line usage, but I don't think you
> are dealing with command lines at that point.)

Right now zipimport doesn't even support __pycache__ (I think). Besides,
zipimport already does a string substitution of os.altsep with os.sep (see
Modules/zipimport.c:90 amongst other places) so it also doesn't care in the
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