[Python-Dev] Issue 13524: subprocess on Windows

[Martin Packman]

On 04/12/2011, Tim Golden <mail at timgolden.me.uk> wrote:
>
> Someone raised issue13524 yesterday to illustrate that a
> subprocess will crash immediately if an environment block is
> passed which does not contain a valid SystemRoot environment
> variable.
...
> 2) Add a doc warning (ironically, considering the recent to-and-fro
>     on doc warnings in this very module).

There appears to already be such a warning, added because of a similar
earlier bug:

<http://bugs.python.org/issue3440>

Really this is a problem with the subprocess api making a common case
harder to do than necessary. If you read the documentation, you'll get
it right, but that's not ideal:

<http://sourcefrog.net/weblog/software/aesthetics/interface-levels.html>

>From the bug, the problem with the reporter's code is he passes a dict
with the one value he cares about as `env` to subprocess.Popen without
realising that it will prevent the inheriting of the current
environment. Your suggested fix for him also has an issue, it changes
the environment of the parent process without resetting it. Instead
you need something like:

    e = dict(os.environ)
    e['PATH_TO_MY_APPS'] = "path/to/my/apps"

The bzrlib TestCase has a method using subprocess that provides an
`env_changes` argument. With that, it's much easier to override or
remove just one variable without accidentally clearing the current
environment.

Martin