[ python-Bugs-1727024 ] subprocess: unreliability of returncode not clear from docs

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Bugs item #1727024, was opened at 2007-05-28 17:41
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Category: Documentation
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Dan O'Huiginn (ohuiginn)
Assigned to: Nobody/Anonymous (nobody)
Summary: subprocess: unreliability of returncode not clear from docs

Initial Comment:
The docs for the subprocess module (http://docs.python.org/lib/node533.html) give the impression that you can reliably find the return code of a process by checking returncode:

--------
returncode
    The child return code. A None value indicates that the process hasn't terminated yet. A negative value -N indicates that the child was terminated by signal N (Unix only).
--------

But in fact, returncode is only updated when the poll() method is called, and therefore will often be out-of-date. For instance

>>> process=subprocess.Popen('true') #*nix command to do nothing
>>> process.returncode
>>> process.returncode==None
True
>>> process.poll()
0
>>> process.returncode
0


As far as I can see, it is always better to use poll() to check the status or return code of a subprocess. It might be good to either remove returncode from the docs entirely, or at least to explain that it won't always be correct.

[incidentally, having returncode/poll() give None for a running process, and 0 for a process that has exited successfully, seems like a recipe for generating bugs. But I guess it's too late to do anything about that now]


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