[issue12877] Popen(...).stdout.seek(...) throws Illegal seek

2011-09-01 Thread Jonas H. 

New submission from Jonas H. jo...@lophus.org:

from subprocess import Popen, PIPE
p = Popen(['ls'], stdout=PIPE)
p.wait()
p.stdout.seek(0)


Traceback (most recent call last):
  File t.py, line 5, in module
p.stdout.seek(0)
IOError: [Errno 29] Illegal seek

Python 2.7.2, Arch Linux x86-64 (Kernel 3.0)

--
messages: 143323
nosy: jonash
priority: normal
severity: normal
status: open
title: Popen(...).stdout.seek(...) throws Illegal seek
versions: Python 2.7

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[issue12877] Popen(...).stdout.seek(...) throws Illegal seek

2011-09-01 Thread Nadeem Vawda 

Nadeem Vawda nadeem.va...@gmail.com added the comment:

This is expected behaviour - you cannot seek on a pipe.

--
nosy: +nadeem.vawda

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[issue12877] Popen(...).stdout.seek(...) throws Illegal seek

2011-09-01 Thread STINNER Victor 

STINNER Victor victor.stin...@haypocalc.com added the comment:

stdout is a PIPE. You cannot seek in a PIPE.

Write stdout into a file, or use maybe BytesIO or StringIO?

--
nosy: +haypo
resolution:  - invalid
status: open - closed

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[issue12877] Popen(...).stdout.seek(...) throws Illegal seek

2011-09-01 Thread Jonas H. 

Jonas H. jo...@lophus.org added the comment:

Why does it have a 'seek' method then?

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[issue12877] Popen(...).stdout.seek(...) throws Illegal seek

2011-09-01 Thread STINNER Victor 

STINNER Victor victor.stin...@haypocalc.com added the comment:

 Why does it have a 'seek' method then?

Python doesn't remove a method if the operation is forbidden. For example, 
Python doesn't remove write() method if the file is read only, and it doesn't 
remove most methods after close().

I prefer to have files with always the same API. I think that it's more 
practical.

Note: Text files of the io module (TextIOWrapper) raises 
io.UnsupportedOperation on seek() if the file is not seekable (e.g. if the file 
is a pipe):

$ ~/prog/python/default/python 
Python 3.3.0a0 (default:d26c7b18fc9d, Jul 22 2011, 12:04:31) 
 import os
 a,b=os.pipe()
 f=os.fdopen(a, 'rb')
 f
_io.BufferedReader name=3
 f.seek(0)
Traceback (most recent call last):
  File stdin, line 1, in module
IOError: [Errno 29] Illegal seek

 f2=os.fdopen(a, 'r')
 f2
_io.TextIOWrapper name=3 mode='r' encoding='UTF-8'
 f2.seek(0)
Traceback (most recent call last):
  File stdin, line 1, in module
io.UnsupportedOperation: underlying stream is not seekable

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