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)

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

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

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

Why does it have a 'seek' method then?

> 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