[python-win32] regarding invoking command prompt using python

[Tim Roberts]

Vernon Cole wrote:
> Tim:
>   Okay, explain this...
> C:\BZR\sterling\HL7>c:\python26\python.exe
> Python 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit
> (Intel)] on
> win32
> Type "help", "copyright", "credits" or "license" for more information.
> >>> import os
> >>> os.putenv('x','test')
> >>> print os.getenv('x')
> zzz
> >>> exit()
>
> C:\BZR\sterling\HL7>set x
> x=zzz
>
> C:\BZR\sterling\HL7>

Depending on your point of view, that's either a usage problem or a
design flaw in the "os" module.  On Windows, os.putenv actually calls
the operating system service that changes the current environment, but
os.getenv does not: it just reads from the os.environ dictionary, which
is a snapshot of the environment taken at startup time..  If you had
changed the variable by saying this:
    os.environ['x'] = 'test'
(which is the Pythonic way to do it), you'd find that both the process
environment and Python's snapshot of the environment were updated, and
things would work as I said.  By calling os.putenv, the process
environment is updated, but the Python snapshot is bypassed.  You can
see that os.putenv really does update the current process environment:

    C:\tmp>python
    Python 2.4.1 (#65, Mar 30 2005, 09:13:57) [MSC v.1310 32 bit
    (Intel)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import os, win32api
    >>> os.environ['x']
    'zzz'
    >>> os.getenv('x')
    'zzz'
    >>> win32api.GetEnvironmentVariable('x')
    'zzz'
    >>> os.putenv('x','test')
    >>> os.environ['x']
    'zzz'
    >>> os.getenv('x')
    'zzz'
    >>> win32api.GetEnvironmentVariable('x')
    'test'
    >>> os.system('set x')
    x=test
    0
    >>>

Compare this to:

    C:\tmp>python
    Python 2.4.1 (#65, Mar 30 2005, 09:13:57) [MSC v.1310 32 bit
    (Intel)] on win32
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import os, win32api
    >>> os.environ['x']
    'zzz'
    >>> os.environ['x'] = 'test'
    >>> os.environ['x']
    'test'
    >>> os.getenv('x')
    'test'
    >>> win32api.GetEnvironmentVariable('x')
    'test'
    >>> os.system('set x')
    x=test
    0
    >>>

The takeaway here is, basically, don't ever call os.putenv.  Instead,
use os.environ.

-- 
Tim Roberts, timr at probo.com
Providenza & Boekelheide, Inc.