Popen(...).stdout.seek(...) throws "Illegal seek"

[jonashaag]

BPO	12877
Nosy	@vstinner, @jonashaag

^{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 = None
closed_at = <Date 2011-09-01.13:44:32.166>
created_at = <Date 2011-09-01.13:38:32.620>
labels = ['invalid']
title = 'Popen(...).stdout.seek(...) throws "Illegal seek"'
updated_at = <Date 2011-09-01.14:28:15.608>
user = 'https://github.com/jonashaag'

bugs.python.org fields:

activity = <Date 2011-09-01.14:28:15.608>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2011-09-01.13:44:32.166>
closer = 'vstinner'
components = []
creation = <Date 2011-09-01.13:38:32.620>
creator = 'jonash'
dependencies = []
files = []
hgrepos = []
issue_num = 12877
keywords = []
message_count = 5.0
messages = ['143323', '143324', '143325', '143328', '143329']
nosy_count = 3.0
nosy_names = ['vstinner', 'nadeem.vawda', 'jonash']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue12877'
versions = ['Python 2.7']

[jonashaag]

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)

[nadeemvawda]

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

[vstinner]

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

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

[jonashaag]

Why does it have a 'seek' method then?

[vstinner]

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