[issue1631] Send output from subprocess.Popen objects to any object with a write() method

[Guido van Rossum]

Changes by Guido van Rossum:


--
status: pending - closed

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[issue1631] Send output from subprocess.Popen objects to any object with a write() method

[Nishkar Grover]

New submission from Nishkar Grover:

It would be nice if we could send output from subprocess.Popen objects
to any object with a write() method.

Consider the following example, where I'm using Python 2.4.4 (#1, Jun 28
2007, 15:10:17, GCC 3.4.3 on linux2)...


 fh = open('/tmp/file.txt', 'w')
 cmdObj = subprocess.Popen('uname -a', shell=True, stdout=fh)
 fh.close()
 fh = open('/tmp/file.txt', 'r')
 fh.read()
'Linux northstar 2.6.9-42.0.2.IT111843.1.ELsmp #1 SMP Fri May 11
18:50:54 EDT 2007 x86_64 x86_64 x86_64 GNU/Linux\n'


That demonstrates that I can successfully send the stdout output from my
subprocess.Popen object to a file handle.

I tried to send that output to a custom object that has a write() method...


 class Foo(object):
... def write(self, msg):
... sys.stdout.write('*** %s ***' % msg.rstrip() + os.linesep)
... sys.stdout.flush()
... def close(self):
... pass
...

 foo = Foo()

 cmdObj = subprocess.Popen('uname -a', shell=True, stdout=foo)
Traceback (most recent call last):
  File stdin, line 1, in ?
  File /rel/lang/python/2.4.4-4/lib/python2.4/subprocess.py, line 534,
in __init__
(p2cread, p2cwrite,
  File /rel/lang/python/2.4.4-4/lib/python2.4/subprocess.py, line 840,
in _get_handles
c2pwrite = stdout.fileno()
AttributeError: 'Foo' object has no attribute 'fileno'


so I gave it a fileno() method that returns None...


 class Foo(object):
... def write(self, msg):
... sys.stdout.write('*** %s ***' % msg.rstrip() + os.linesep)
... sys.stdout.flush()
... def close(self):
... pass
... def fileno(self):
... return None
...

 foo = Foo()

 cmdObj = subprocess.Popen('uname -a', shell=True, stdout=foo)
 Linux northstar 2.6.9-42.0.2.IT111843.1.ELsmp #1 SMP Fri May 11
18:50:54 EDT 2007 x86_64 x86_64 x86_64 GNU/Linux


Notice that the output went straight to the console and bypassed my
'foo' object's write() method.

--
components: Library (Lib)
messages: 58646
nosy: ngrover
severity: normal
status: open
title: Send output from subprocess.Popen objects to any object with a write() 
method
type: behavior
versions: Python 2.4

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[issue1631] Send output from subprocess.Popen objects to any object with a write() method

[Christian Heimes]

Christian Heimes added the comment:

It's not going to be easy and there is also no point to implement the
feature. The subprocess module requires either a real file or a PIPE. A
real file is needed because the subprocess module uses some low level
operation system functions for speed efficiency. It writes data directly
to a file descriptor (the number returned by fileno()).

In your case it seems have to interpreted None as 0 which is - by
accident - the file descriptor of stdout.

For small amounts of data use Popen(..., stdout=subprocess.PIPE) and use
out, err = communicate([stdin]), for large amounts of data you should
use a temporary file (tempfile module).

In your case please use the platform module instead of uname :)

--
nosy: +tiran
resolution:  - wont fix
status: open - pending

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