[Python-Dev] Update for os.path.expandvars bug 49589

[Behrang Dadsetan]

Here follows the comment I just 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 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 
BDFL.

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), or 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 documentation get updated.

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

So my view point-> same interface, same documentation

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:
             break
         (i, j) = quotearea.span(0)
         quoteareas.append((i, j))
         i = j
     i = 0
     while 1:
         m = _varprog.search(path, i)
         if not m:
             break
         i, j = m.span(0)
         insidequotes=None
         for (quotebegin, quoteend) in quoteareas:
             if quotebegin < i and quoteend > i:
                 insidequotes=1
                 break
         if insidequotes:
             i = j
             continue
         name = m.group(1)
         if name[:1] == '$':
             path = path[:i] + '$' + path[j:]
             i = i + 1
         else:
             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
             else:
                 i = j
     return path

Regards, Ben.