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:mod:`popen2` --- Subprocesses with accessible I/O streams
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==========================================================
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:synopsis: Subprocesses with accessible standard I/O streams.
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.. sectionauthor:: Drew Csillag <drew_csillag@geocities.com>
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This module is obsolete. Use the :mod:`subprocess` module. Check
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especially the :ref:`subprocess-replacements` section.
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This module allows you to spawn processes and connect to their
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input/output/error pipes and obtain their return codes under Unix and Windows.
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The :mod:`subprocess` module provides more powerful facilities for spawning new
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processes and retrieving their results. Using the :mod:`subprocess` module is
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preferable to using the :mod:`popen2` module.
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The primary interface offered by this module is a trio of factory functions.
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For each of these, if *bufsize* is specified, it specifies the buffer size for
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the I/O pipes. *mode*, if provided, should be the string ``'b'`` or ``'t'``; on
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Windows this is needed to determine whether the file objects should be opened in
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binary or text mode. The default value for *mode* is ``'t'``.
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On Unix, *cmd* may be a sequence, in which case arguments will be passed
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directly to the program without shell intervention (as with :func:`os.spawnv`).
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If *cmd* is a string it will be passed to the shell (as with :func:`os.system`).
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The only way to retrieve the return codes for the child processes is by using
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the :meth:`poll` or :meth:`wait` methods on the :class:`Popen3` and
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:class:`Popen4` classes; these are only available on Unix. This information is
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not available when using the :func:`popen2`, :func:`popen3`, and :func:`popen4`
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functions, or the equivalent functions in the :mod:`os` module. (Note that the
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tuples returned by the :mod:`os` module's functions are in a different order
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from the ones returned by the :mod:`popen2` module.)
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.. function:: popen2(cmd[, bufsize[, mode]])
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Executes *cmd* as a sub-process. Returns the file objects ``(child_stdout,
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.. function:: popen3(cmd[, bufsize[, mode]])
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Executes *cmd* as a sub-process. Returns the file objects ``(child_stdout,
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child_stdin, child_stderr)``.
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.. function:: popen4(cmd[, bufsize[, mode]])
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Executes *cmd* as a sub-process. Returns the file objects
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``(child_stdout_and_stderr, child_stdin)``.
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On Unix, a class defining the objects returned by the factory functions is also
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available. These are not used for the Windows implementation, and are not
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available on that platform.
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.. class:: Popen3(cmd[, capturestderr[, bufsize]])
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This class represents a child process. Normally, :class:`Popen3` instances are
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created using the :func:`popen2` and :func:`popen3` factory functions described
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If not using one of the helper functions to create :class:`Popen3` objects, the
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parameter *cmd* is the shell command to execute in a sub-process. The
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*capturestderr* flag, if true, specifies that the object should capture standard
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error output of the child process. The default is false. If the *bufsize*
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parameter is specified, it specifies the size of the I/O buffers to/from the
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.. class:: Popen4(cmd[, bufsize])
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Similar to :class:`Popen3`, but always captures standard error into the same
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file object as standard output. These are typically created using
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Popen3 and Popen4 Objects
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-------------------------
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Instances of the :class:`Popen3` and :class:`Popen4` classes have the following
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.. method:: Popen3.poll()
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Returns ``-1`` if child process hasn't completed yet, or its status code
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(see :meth:`wait`) otherwise.
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.. method:: Popen3.wait()
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Waits for and returns the status code of the child process. The status code
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encodes both the return code of the process and information about whether it
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exited using the :c:func:`exit` system call or died due to a signal. Functions
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to help interpret the status code are defined in the :mod:`os` module; see
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section :ref:`os-process` for the :func:`W\*` family of functions.
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The following attributes are also available:
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.. attribute:: Popen3.fromchild
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A file object that provides output from the child process. For :class:`Popen4`
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instances, this will provide both the standard output and standard error
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.. attribute:: Popen3.tochild
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A file object that provides input to the child process.
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.. attribute:: Popen3.childerr
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A file object that provides error output from the child process, if
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*capturestderr* was true for the constructor, otherwise ``None``. This will
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always be ``None`` for :class:`Popen4` instances.
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.. attribute:: Popen3.pid
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The process ID of the child process.
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.. _popen2-flow-control:
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Any time you are working with any form of inter-process communication, control
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flow needs to be carefully thought out. This remains the case with the file
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objects provided by this module (or the :mod:`os` module equivalents).
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When reading output from a child process that writes a lot of data to standard
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error while the parent is reading from the child's standard output, a deadlock
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can occur. A similar situation can occur with other combinations of reads and
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writes. The essential factors are that more than :const:`_PC_PIPE_BUF` bytes
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are being written by one process in a blocking fashion, while the other process
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is reading from the first process, also in a blocking fashion.
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.. Example explanation and suggested work-arounds substantially stolen
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from Martin von Löwis:
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https://mail.python.org/pipermail/python-dev/2000-September/009460.html
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There are several ways to deal with this situation.
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The simplest application change, in many cases, will be to follow this model in
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r, w, e = popen2.popen3('python slave.py')
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with code like this in the child::
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# note that each of these print statements
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# writes a single long string
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print >>sys.stderr, 400 * 'this is a test\n'
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os.close(sys.stderr.fileno())
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print >>sys.stdout, 400 * 'this is another test\n'
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In particular, note that ``sys.stderr`` must be closed after writing all data,
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or :meth:`readlines` won't return. Also note that :func:`os.close` must be
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used, as ``sys.stderr.close()`` won't close ``stderr`` (otherwise assigning to
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``sys.stderr`` will silently close it, so no further errors can be printed).
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Applications which need to support a more general approach should integrate I/O
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over pipes with their :func:`select` loops, or use separate threads to read each
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of the individual files provided by whichever :func:`popen\*` function or
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:class:`Popen\*` class was used.
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Module :mod:`subprocess`
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Module for spawning and managing subprocesses.