[ python-Bugs-1220113 ] subprocess call() helper should close stdin if PIPE

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Bugs item #1220113, was opened at 2005-06-14 15:04
Message generated for change (Comment added) made by zenzen
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Category: None
Group: None
Status: Closed
Resolution: Rejected
Priority: 5
Submitted By: Stuart Bishop (zenzen)
Assigned to: Peter Åstrand (astrand)
Summary: subprocess call() helper should close stdin if PIPE

Initial Comment:
The following code snippet should die instead of hang.

>>> from subprocess import call, PIPE
>>> rv = call(['/usr/bin/bzip2', '-c'], stdout=PIPE,
stdin=PIPE)

It makes no sense not to close stdin if it is PIPE
because the stream cannot be accessed.

The use case for this is ensuring a subprocess that
detects if it is connected to a terminal or not runs in
'batch' mode, and that it will die instead of hang if
it unexpectidly attempts to read from stdin.

Workaround is to use Popen instead.

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>Comment By: Stuart Bishop (zenzen)
Date: 2005-06-22 16:12

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I can't think of any uses cases for wanting to create an
inaccessible pipe and give it to the child. 

Wanting to pass a closed file handle is common. It is needed
when calling a program that behaves differently if its stdin
is a terminal or not. Or when you simply would prefer the
subprocess to die if it attempts to read from its stdin
rather than block.

Using Popen instead of call is s simpler workaround than
creating and closing a file descriptor and passing it in.

Perhaps what is needed is a new constant, subprocess.CLOSED
which creates a new file descriptor and closes it? This
would be useful for Popen too, allowing call() to remain a
think and trivially documented wrapper?

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Comment By: Peter Åstrand (astrand)
Date: 2005-06-22 02:08

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>It makes no sense not to close stdin if it is PIPE
>because the stream cannot be accessed

True, but what if you actually *want* to create an
inaccessible pipe, and give it to the child?

Currently, the call() wrapper is *very* short and simple. I
think this is very good. For example, it allows us to
document it in two sentences. You get what you ask for: If
you use call() with strange arguments, you'll get a somewhat
strange behavíour. I see no point in introducing lots of
sanity checks in the wrapper functions. 

>The use case for this is ensuring a subprocess that
>detects if it is connected to a terminal or not runs in
>batch' mode, and that it will die instead of hang if
>it unexpectidly attempts to read from stdin

I'm not sure I understand what you want, but if you want to
have stdin connected to a closed file descriptor, just pass one:

>>> from subprocess import call, PIPE
>>> rv = call(['/usr/bin/bzip2', '-c'], stdout=PIPE,
stdin=4711)

(Of course, you should verify that 4711 is unused.)

If you don't agree with me, post to python-dev for discussion. 

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