1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
|
: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This module is obsolete.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 other 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.
|
