ActivePython 2.2.2 Build 224 (ActiveState Corp.) based 
on
Python 2.2.2 (#37, Nov 26 2002, 10:24:37) [MSC 32 bit 
(Intel)] on win32
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information.
>>> import os
>>> os.listdir('c:\IMPOSSIBLE')
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
WindowsError: [Errno 3] The system cannot find the 
path specified:
'c:\\IMPOSSIBLE/*.*'


This is the wrong exception I would have thought.

AIUI, WindowsError is thrown for uniquely Windows 
types of problems.  Path not found is not unique to 
Windows.

I would have expected an os.error exception to have 
been thrown together with an code of errno.ENOENT.  
This is what happens if I try to open() a non existent file. 

(Caution:  I'm a python newbie and I could easily have 
missed something here -- If so sorry to waste your time)

The deadly part about this bug is that WindowsError 
subclasses os.error, just the code changes (3 instead of 
2). So my   "except os.error"  worked to catch the 
exception ... just the error code seemed wrong.  Took 
be ages to figure it all out.

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Python faithfully relays to you the information returned by the OS in these cases.  I don't think it's planned for Python to try to rationalize the various errno/GetLastError() values based on what it thinks the actual error must have been.

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Yet, if I try to open() a non existent file, this mapping to a 
generic Python exception DOES occur (exception is os.error, 
code is errno.ENOENT)

And just as well it does too because otherwise cross-platform 
coding would be difficult.

Now, if its good enough for open(), why not for os.listdir()?

From the documentation, I got the impression that 
WindowError gets used for Windows specific problems for 
which generic cross-platform handling makes no sense, 
which is why WindowsError simply wraps errno/GetLastError
() and sub-classes the standard exception. 

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WindowsError gets raised when the implementation uses a 
native Win32 API function instead of a standard C library 
function:  no native Win32 API function sets errno, and the 
error code we get is gotten from the Win32 GetLastError() 
function instead..  os.listdir() on Windows uses the Win32  
FindFirstFile()/FindNextFile() API.  That doesn't set errno.  
When a path can't be found by FindFisrtFile(), GetLastError
() returns 3, which is the Windows 
ERROR_PATH_NOT_FOUND.  That's what we tell you.

On Unix systems, the POSIX opendir()/readdir()/closedir() 
API is used.  That does set errno, and we return whatever 
errno it sets there.

On OS2, we use its DosFindFirst()/DosFindNext() API.

Etc.  The reason this isn't uniform is that standard C 
doesn't define a way to list a directory, so how to get it 
done necessarily varies across platforms.  In platform-
specific code, we generally strive to give platform-natural 
error codes. open() is different because standard C defines 
what open() does, so it can be done the same way across 
platfroms.

There isn't "a bug" here, so I'm closing this.  If you want to 
open a feature request asking that error codes be made 
uniform across all platforms, you can do so, but I'll tell you 
it's unlikely to happen -- there are cases where no cross-
platform standard exists, and, e.g., to a Win32 
programmer it wouldn't make sense for listdir() to return a 
POSIX errno code.  It's not impossble for Python to make it 
do so anyway, but Python chose a different way at the start 
and changing now would break backward compatibility.  A 
change here would therefore require a PEP too.