[Python-bugs-list] [ python-Bugs-513572 ] isdir behavior getting odder on UNC path

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Bugs item #513572, was opened at 2002-02-05 18:07
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Category: Python Library
Group: Python 2.2
Status: Open
Resolution: None
Priority: 5
Submitted By: Gary Herron (herron)
Assigned to: Mark Hammond (mhammond)
Summary: isdir behavior getting odder on UNC path

Initial Comment:
It's been documented in earlier version of Python on 
windows that os.path.isdir returns true on a UNC 
directory only if there was an extra backslash at the 
end of the argument.  In Python2.2 (at least on 
windows 2000) it appears that *TWO* extra backslashes 
are needed.

Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit 
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for 
more information.
>>>
>>> import os
>>> os.path.isdir('\\trainer\island')
0
>>> os.path.isdir('\\trainer\island\')
0
>>> os.path.isdir('\\trainer\island\\')
1
>>>

In a perfect world, the first call should return 1, 
but never has.  In older versions of python, the 
second returned 1, but no longer.

In limited tests, appending 2 or more backslashes to 
the end of any pathname returns the correct answer in 
both isfile and isdir.


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>Comment By: Tim Peters (tim_one)
Date: 2002-02-10 10:57

Message:
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Gary, exactly what do you mean by "older versions of 
Python"?  That is, specifically which versions?

The Microsoft stat() function is extremely picky about 
trailing (back)slashes.  For example, if you have a 
directory c:/python, and pass "c:/python/" to the MS stat
(), it claims no such thing exists.  This isn't documented 
by MS, but that's how it works:  a trailing (back)slash is 
required if and only if the path passed in "is a root".  So 
MS stat() doesn't understand "/python/", and doesn't 
understand "d:" either.  The former doesn't tolerate a 
(back)slash, while the latter requires one.

This is impossible for people to keep straight, so after 
1.5.2 Python started removing (back)slashes on its own to 
make MS stat() happy.  The code currently leaves a trailing 
(back)slash alone if and only if one exists, and in 
addition of these obtains:

1) The (back)slash is the only character in the path.
or
2) The path has 3 characters, and the middle one is a colon.

UNC roots don't fit either of those, so do get one (back)
slash chopped off.  However, just as for any other roots, 
the MS stat() refuses to recognize them as valid unless 
they do have a trailing (back)slash.  Indeed, the last time 
I applied a contributed patch to this code, I added a

/* XXX UNC root drives should also be exempted? */

comment there.

However, this explanation doesn't make sense unless by 
"older versions of Python" you mean nothing more recent 
than 1.5.2.  If I'm understanding the source of the 
problem, it should exist in all Pythons after 1.5.2.  So if 
you don't see the same problem in 1.6, 2.0 or 2.1, I'm on 
the wrong track.

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Comment By: Tim Peters (tim_one)
Date: 2002-02-08 15:33

Message:
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BTW, it occurs to me that this *may* be a consequence of 
whatever was done in 2.2 to encode/decode filename strings 
for system calls on Windows.  I didn't follow that, and 
Mark may be the only one who fully understands the details.

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Comment By: Tim Peters (tim_one)
Date: 2002-02-08 15:17

Message:
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Here's the implementation of Windows isdir():

def isdir(path):
.    """Test whether a path is a directory"""
.    try:
.        st = os.stat(path)
.    except os.error:
.        return 0
.    return stat.S_ISDIR(st[stat.ST_MODE])

That is, we return whatever Microsoft's stat() tells us, 
and our code is the same in 2.2 as in 2.1.  I don't have 
Win2K here, and my Win98 box isn't on a Windows network so 
I can't even try real UNC paths here.  Reassigning to MarkH 
in case he can do better on either count.

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Comment By: Guido van Rossum (gvanrossum)
Date: 2002-02-08 14:05

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Tim, I hate to do this to you, but you're the only person I
trust with researching this. (My laptop is currently off the
net again. :-( )


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