Possible fix for Bug 494589 - os.path.expandvars bug

Behrang Dadsetan ben at dadsetan.com
Tue Jul 1 23:47:11 CEST 2003

Here follows the comment I added to bug "[ 494589 ] os.path.expandvars 
deletes things on w32".

It concerns some mismatching (buggy) implementation / docstrings / 
comments for the ntpath.py and dospath.py.

(Especially) As it is my first post as well as bugfix try for any 
open-source project, I am very happy receiving constructiv critic.

tim_one (see other 494589 bug comments) is right. There is plenty of 
dodgy things hiding behind the os.path world, especially when it comes 
to os.path.expandvars()

There are two problems here.
- Mismatch in between the doc strings of the different implementation of 
expandvars and the "official" os.path.expandvars documentation.
- the ntpath and dospath implementations are buggy when compared to 
their comments/docstrings.

About the first problem, the inconsistency created some time ago in 
between the different implementations tasks makes it difficult to choose 
a solution. Everyone will probably agree that all the platform specific 
implementations of expandvars should have the same functionality. The 
one that should be taken over will probably need to be announced by the 

Some rule which should not have let this here happen, and on which I 
believe we all will agree on:
Same interface=same documentation->same functionality

To implement either copy paste exactly the same expandvars definition 
from one platform to another (NT, DOS, POSIX).
Alternatively somehow rather arrange that when there is no specific 
implementation for the platform, a "default" python implementation is 
used on the os.path level.

To maximize the fruits of my small work, I would of course prefer that 
the version below becomes the standard and that the Library 
documentation get updated.

To be complete, shall the documentation remain unchanged (which will 
probablz be chosen) and the implementation of dos and nt gets adapted 
(copied from posix). But I really feel its docstring and its 
documentation should be in line with the rest of the implementations.

For the second problem - as of now a real bug whatever we decide, I 
wrote within this comment (hereafter) a new expandvars version which 
fits the docstring documentation of dospath.py and the comments of 
ntpath.py. Sorry you will be getting no patch from me at the moment 
since sourceforge's anonymous CVS access does not like me. Please note 
that my version borrows alot from the posixpath.py implementation and my 
changes are the ones of a python amateur who is open to critic.

#expandvars() implementation
_varprog = None
_findquotes = None
def expandvars(path):
     """Expand paths containing shell variable substitutions.
     The following rules apply:
         - no expansion within single quotes
         - no escape character, except for '$$' which is translated into '$'
         - ${varname} is accepted.
         - varnames can be made out of letters, digits and the character 
     global _varprog, _findquotes
     if '$' not in path:
         return path
     if not _varprog:
         import re
         _varprog = re.compile(r'\$(\w+|\{[^}]*\}|\$)')
         _findquotes = re.compile("'.*?'")
     quoteareas = []
     i = 0
     while 1:
         quotearea = _findquotes.search(path, i)
         if not quotearea:
         (i, j) = quotearea.span(0)
         quoteareas.append((i, j))
         i = j
     i = 0
     while 1:
         m = _varprog.search(path, i)
         if not m:
         i, j = m.span(0)
         for (quotebegin, quoteend) in quoteareas:
             if quotebegin < i and quoteend > i:
         if insidequotes:
             i = j
         name = m.group(1)
         if name[:1] == '$':
             path = path[:i] + '$' + path[j:]
             i = i + 1
             if name[:1] == '{' and name[-1:] == '}':
                 name = name[1:-1]
             if os.environ.has_key(name):
                 tail = path[j:]
                 path = path[:i] + os.environ[name]
                 i = len(path)
                 path = path + tail
                 i = j
     return path

Regards, Ben.

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