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+
+: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+ This module is obsolete.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.
+