\section{\module{popen2} ---
         Subprocesses with accessible I/O streams}

\declaremodule{standard}{popen2}
  \platform{Unix, Windows}
\modulesynopsis{Subprocesses with accessible standard I/O streams.}
\sectionauthor{Drew Csillag}{drew_csillag@geocities.com}


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

Note that starting with Python 2.0, this functionality is available
using functions from the \refmodule{os} module which have the same
names as the factory functions here, but the order of the return
values is more intuitive in the \refmodule{os} module variants.

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

\begin{funcdesc}{popen2}{cmd\optional{, bufsize\optional{, mode}}}
Executes \var{cmd} as a sub-process.  Returns the file objects
\code{(\var{child_stdout}, \var{child_stdin})}.
\end{funcdesc}

\begin{funcdesc}{popen3}{cmd\optional{, bufsize\optional{, mode}}}
Executes \var{cmd} as a sub-process.  Returns the file objects
\code{(\var{child_stdout}, \var{child_stdin}, \var{child_stderr})}.
\end{funcdesc}

\begin{funcdesc}{popen4}{cmd\optional{, bufsize\optional{, mode}}}
Executes \var{cmd} as a sub-process.  Returns the file objects
\code{(\var{child_stdout_and_stderr}, \var{child_stdin})}.
\versionadded{2.0}
\end{funcdesc}


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.

\begin{classdesc}{Popen3}{cmd\optional{, capturestderr\optional{, bufsize}}}
This class represents a child process.  Normally, \class{Popen3}
instances are created using the \function{popen2()} and
\function{popen3()} factory functions described above.

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

\begin{classdesc}{Popen4}{cmd\optional{, bufsize}}
Similar to \class{Popen3}, but always captures standard error into the
same file object as standard output.  These are typically created
using \function{popen4()}.
\versionadded{2.0}
\end{classdesc}


\subsection{Popen3 and Popen4 Objects \label{popen3-objects}}

Instances of the \class{Popen3} and \class{Popen4} classes have the
following methods:

\begin{methoddesc}{poll}{}
Returns \code{-1} if child process hasn't completed yet, or its return 
code otherwise.
\end{methoddesc}

\begin{methoddesc}{wait}{}
Waits for and returns the return code of the child process.
\end{methoddesc}


The following attributes are also available: 

\begin{memberdesc}{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.
\end{memberdesc}

\begin{memberdesc}{tochild}
A file object that provides input to the child process.
\end{memberdesc}

\begin{memberdesc}{childerr}
Where the standard error from the child process goes is
\var{capturestderr} was true for the constructor, or \code{None}.
This will always be \code{None} for \class{Popen4} instances.
\end{memberdesc}

\begin{memberdesc}{pid}
The process ID of the child process.
\end{memberdesc}
