cpython/Doc/library/popen2.rst

:mod:`popen2` --- Subprocesses with accessible I/O streams
==========================================================

.. module:: popen2
   :synopsis: Subprocesses with accessible standard I/O streams.
   :deprecated:
.. sectionauthor:: Drew Csillag <drew_csillag@geocities.com>


.. deprecated:: 2.6
   This module is obsolete.  Use the :mod:`subprocess` module.  Check
   especially the :ref:`subprocess-replacements` section.

This module allows you to spawn processes and connect to their
input/output/error pipes and obtain their return codes under Unix and Windows.

The :mod:`subprocess` module provides more powerful facilities for spawning new
processes and retrieving their results.  Using the :mod:`subprocess` module is
preferable to using the :mod:`popen2` module.

The primary interface offered by this module is a trio of factory functions.
For each of these, if *bufsize* is specified,  it specifies the buffer size for
the I/O pipes.  *mode*, if provided, should be the string ``'b'`` or ``'t'``; on
Windows this is needed to determine whether the file objects should be opened in
binary or text mode.  The default value for *mode* is ``'t'``.

On Unix, *cmd* may be a sequence, in which case arguments will be passed
directly to the program without shell intervention (as with :func:`os.spawnv`).
If *cmd* is a string it will be passed to the shell (as with :func:`os.system`).

The only way to retrieve the return codes for the child processes is by using
the :meth:`poll` or :meth:`wait` methods on the :class:`Popen3` and
:class:`Popen4` classes; these are only available on Unix.  This information is
not available when using the :func:`popen2`, :func:`popen3`, and :func:`popen4`
functions, or the equivalent functions in the :mod:`os` module. (Note that the
tuples returned by the :mod:`os` module's functions are in a different order
from the ones returned by the :mod:`popen2` module.)


.. function:: popen2(cmd[, bufsize[, mode]])

   Executes *cmd* as a sub-process.  Returns the file objects ``(child_stdout,
   child_stdin)``.


.. function:: popen3(cmd[, bufsize[, mode]])

   Executes *cmd* as a sub-process.  Returns the file objects ``(child_stdout,
   child_stdin, child_stderr)``.


.. function:: popen4(cmd[, bufsize[, mode]])

   Executes *cmd* as a sub-process.  Returns the file objects
   ``(child_stdout_and_stderr, child_stdin)``.

   .. versionadded:: 2.0

On Unix, a class defining the objects returned by the factory functions is also
available.  These are not used for the Windows implementation, and are not
available on that platform.


.. class:: Popen3(cmd[, capturestderr[, bufsize]])

   This class represents a child process.  Normally, :class:`Popen3` instances are
   created using the :func:`popen2` and :func:`popen3` factory functions described
   above.

   If not using one of the helper functions to create :class:`Popen3` objects, the
   parameter *cmd* is the shell command to execute in a sub-process.  The
   *capturestderr* flag, if true, specifies that the object should capture standard
   error output of the child process. The default is false.  If the *bufsize*
   parameter is specified, it specifies the size of the I/O buffers to/from the
   child process.


.. class:: Popen4(cmd[, bufsize])

   Similar to :class:`Popen3`, but always captures standard error into the same
   file object as standard output.  These are typically created using
   :func:`popen4`.

   .. versionadded:: 2.0


.. _popen3-objects:

Popen3 and Popen4 Objects
-------------------------

Instances of the :class:`Popen3` and :class:`Popen4` classes have the following
methods:


.. method:: Popen3.poll()

   Returns ``-1`` if child process hasn't completed yet, or its status code
   (see :meth:`wait`) otherwise.


.. method:: Popen3.wait()

   Waits for and returns the status code of the child process.  The status code
   encodes both the return code of the process and information about whether it
   exited using the :cfunc:`exit` system call or died due to a signal.  Functions
   to help interpret the status code are defined in the :mod:`os` module; see
   section :ref:`os-process` for the :func:`W\*` family of functions.

The following attributes are also available:


.. attribute:: Popen3.fromchild

   A file object that provides output from the child process.  For :class:`Popen4`
   instances, this will provide both the standard output and standard error
   streams.


.. attribute:: Popen3.tochild

   A file object that provides input to the child process.


.. attribute:: Popen3.childerr

   A file object that provides error output from the child process, if
   *capturestderr* was true for the constructor, otherwise ``None``.  This will
   always be ``None`` for :class:`Popen4` instances.


.. attribute:: Popen3.pid

   The process ID of the child process.


.. _popen2-flow-control:

Flow Control Issues
-------------------

Any time you are working with any form of inter-process communication, control
flow needs to be carefully thought out.  This remains the case with the file
objects provided by this module (or the :mod:`os` module equivalents).

When reading output from a child process that writes a lot of data to standard
error while the parent is reading from the child's standard output, a deadlock
can occur.  A similar situation can occur with other combinations of reads and
writes.  The essential factors are that more than :const:`_PC_PIPE_BUF` bytes
are being written by one process in a blocking fashion, while the other process
is reading from the first process, also in a blocking fashion.

.. Example explanation and suggested work-arounds substantially stolen
   from Martin von Löwis:
   http://mail.python.org/pipermail/python-dev/2000-September/009460.html

There are several ways to deal with this situation.

The simplest application change, in many cases, will be to follow this model in
the parent process::

   import popen2

   r, w, e = popen2.popen3('python slave.py')
   e.readlines()
   r.readlines()
   r.close()
   e.close()
   w.close()

with code like this in the child::

   import os
   import sys

   # note that each of these print statements
   # writes a single long string

   print >>sys.stderr, 400 * 'this is a test\n'
   os.close(sys.stderr.fileno())
   print >>sys.stdout, 400 * 'this is another test\n'

In particular, note that ``sys.stderr`` must be closed after writing all data,
or :meth:`readlines` won't return.  Also note that :func:`os.close` must be
used, as ``sys.stderr.close()`` won't close ``stderr`` (otherwise assigning to
``sys.stderr`` will silently close it, so no further errors can be printed).

Applications which need to support a more general approach should integrate I/O
over pipes with their :func:`select` loops, or use separate threads to read each
of the individual files provided by whichever :func:`popen\*` function or
:class:`Popen\*` class was used.


.. seealso::

   Module :mod:`subprocess`
      Module for spawning and managing subprocesses.