Improve subprocess module usage

[mmokrejs]

BPO	3235
Nosy	@loewis, @birkenfeld

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/birkenfeld'
closed_at = <Date 2008-07-01.20:00:22.246>
created_at = <Date 2008-06-29.09:26:13.726>
labels = ['docs', 'performance']
title = 'Improve subprocess module usage'
updated_at = <Date 2008-07-05.16:39:38.860>
user = 'https://bugs.python.org/mmokrejs'

bugs.python.org fields:

activity = <Date 2008-07-05.16:39:38.860>
actor = 'georg.brandl'
assignee = 'georg.brandl'
closed = True
closed_date = <Date 2008-07-01.20:00:22.246>
closer = 'georg.brandl'
components = ['Documentation']
creation = <Date 2008-06-29.09:26:13.726>
creator = 'mmokrejs'
dependencies = []
files = []
hgrepos = []
issue_num = 3235
keywords = []
message_count = 7.0
messages = ['68943', '68944', '68956', '68959', '69062', '69111', '69284']
nosy_count = 4.0
nosy_names = ['loewis', 'georg.brandl', 'mmokrejs', 'dwblas']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'resource usage'
url = 'https://bugs.python.org/issue3235'
versions = ['Python 2.5']

[mmokrejs]

Although I do appreciate that you try to improve python it is not clear
to me from http://docs.python.org/lib/module-subprocess.html:

1. Why the old functions have been deprecated
2. I can pipe together two processes. But how can I use pipe to feed the
   first process of the two? The example at
   http://docs.python.org/lib/node535.html is not sufficient. Show how to
   bind together cat(1) and tr(1). Cat should read from STDIN and pass the
   same to STDOUT, being read by tr(1).
3. Nowhere is explained what this magic really does:
   output = p2.communicate()[0]
   I figured out the result is e.g. a string of '\n' separated lines.
   What is not clear whether at this very moment the processes get
   started and whether one can still write on input to them. It seems
   not, the p1.stdin has to be already populated with data.
   Thanks.

[mmokrejs]

Please link to http://www.python.org/dev/peps/pep-0324/ from the docs
webpage at least.

[dwblas]

See if Doug Hellman's module of the week helps any
http://blog.doughellmann.com/2007/07/pymotw-subprocess.html I plan on
asking him if we can include some of his examples in the Python 3000
docs. Subprocess is new enough and gets enough questions on the forums
that I think we should be as detailed as possible and include more
examples than there are in other doc/tools examples.

[loewis]

1. The documentation never gives a rationale for deprecating things. To
   find out why they are deprecated, you'll typically have to research what
   checkins to a code base have been made that added the deprecation, and
   perhaps also ask on mailing lists. For the regular developer, the
   rationale for deprecation should be irrelevant, as he needs to change
   his code to drop usage of the deprecated API, anyway.

2. You really need to read all of the documentation, in particular the
   section that talks about stdin,stdout,stderr in

http://docs.python.org/lib/node528.html

In the example, stdin is inherited from the parent process. Read the
documentation to find out how to send data to the first process.

3. It is not true that this is nowhere explained.

http://docs.python.org/lib/node532.html

explains precisely what the semantics of communicate is.

[birkenfeld]

Added a link to PEP-324 in r64625. The other issues were explained by
Martin; it will be easier to read the subprocess docs in 2.6 where
they're all on one page.

[mmokrejs]

Georg, but would you please improve the docs explaining what communicate
really does? The syntax is nice but I don't see how can I use poll()
described in the same above to use that. Providing Examples section
would be the best. Of course I don't mind if the examples come from Doug
Hellman or not, as mentioned in msg68956. ;-) Or how about placing a
link to that page
(http://blog.doughellmann.com/2007/07/pymotw-subprocess.html)?

Please reopen unless better docs are available. I don't care about 2.6
either, sorry.

[birkenfeld]

I don't understand the comment about poll().

In any case, I plan on referencing Doug's PyMOTW pages for all modules
he does, since they are indeed excellent tutorials.