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PEP 324: popen5 - New POSIX process module

P: n/a

There's a new PEP available:

PEP 324: popen5 - New POSIX process module

A copy is included below. Comments are appreciated.

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PEP: 324
Title: popen5 - New POSIX process module
Version: $Revision: 1.4 $
Last-Modified: $Date: 2004/01/03 10:32:53 $
Author: Peter Astrand <as*****@lysator.liu.se>
Status: Draft
Type: Standards Track (library)
Created: 19-Nov-2003
Content-Type: text/plain
Python-Version: 2.4
Abstract

This PEP describes a new module for starting and communicating
with processes on POSIX systems.
Motivation

Starting new processes is a common task in any programming
language, and very common in a high-level language like Python.
Good support for this task is needed, because:

- Inappropriate functions for starting processes could mean a
security risk: If the program is started through the shell, and
the arguments contain shell meta characters, the result can be
disastrous. [1]

- It makes Python an even better replacement language for
over-complicated shell scripts.

Currently, Python has a large number of different functions for
process creation. This makes it hard for developers to choose.

The popen5 modules provides the following enhancements over
previous functions:

- One "unified" module provides all functionality from previous
functions.

- Cross-process exceptions: Exceptions happening in the child
before the new process has started to execute are re-raised in
the parent. This means that it's easy to handle exec()
failures, for example. With popen2, for example, it's
impossible to detect if the execution failed.

- A hook for executing custom code between fork and exec. This
can be used for, for example, changing uid.

- No implicit call of /bin/sh. This means that there is no need
for escaping dangerous shell meta characters.

- All combinations of file descriptor redirection is possible.
For example, the "python-dialog" [2] needs to spawn a process
and redirect stderr, but not stdout. This is not possible with
current functions, without using temporary files.

- With popen5, it's possible to control if all open file
descriptors should be closed before the new program is
executed.

- Support for connecting several subprocesses (shell "pipe").

- Universal newline support.

- A communicate() method, which makes it easy to send stdin data
and read stdout and stderr data, without risking deadlocks.
Most people are aware of the flow control issues involved with
child process communication, but not all have the patience or
skills to write a fully correct and deadlock-free select loop.
This means that many Python applications contain race
conditions. A communicate() method in the standard library
solves this problem.
Rationale

The following points summarizes the design:

- popen5 was based on popen2, which is tried-and-tested.

- The factory functions in popen2 have been removed, because I
consider the class constructor equally easy to work with.

- popen2 contains several factory functions and classes for
different combinations of redirection. popen5, however,
contains one single class. Since popen5 supports 12 different
combinations of redirection, providing a class or function for
each of them would be cumbersome and not very intuitive. Even
with popen2, this is a readability problem. For example, many
people cannot tell the difference between popen2.popen2 and
popen2.popen4 without using the documentation.

- One small utility function is provided: popen5.run(). It aims
to be an enhancement over os.system(), while still very easy to
use:

- It does not use the Standard C function system(), which has
limitations.

- It does not call the shell implicitly.

- No need for quoting; using a variable argument list.

- The return value is easier to work with.

- The "preexec" functionality makes it possible to run arbitrary
code between fork and exec. One might ask why there are special
arguments for setting the environment and current directory, but
not for, for example, setting the uid. The answer is:

- Changing environment and working directory is considered
fairly common.

- Old functions like spawn() has support for an
"env"-argument.

- env and cwd are considered quite cross-platform: They make
sense even on Windows.

- No MS Windows support is available, currently. To be able to
provide more functionality than what is already available from
the popen2 module, help from C modules is required.
Specification

This module defines one class called Popen:

class Popen(args, bufsize=0, argv0=None,
stdin=None, stdout=None, stderr=None,
preexec_fn=None, preexec_args=(), close_fds=0,
cwd=None, env=None, universal_newlines=0)

Arguments are:

- args should be a sequence of program arguments. The program to
execute is normally the first item in the args sequence, but can
be explicitly set by using the argv0 argument. The Popen class
uses os.execvp() to execute the child program.

- bufsize, if given, has the same meaning as the corresponding
argument to the built-in open() function: 0 means unbuffered, 1
means line buffered, any other positive value means use a buffer
of (approximately) that size. A negative bufsize means to use
the system default, which usually means fully buffered. The
default value for bufsize is 0 (unbuffered).

- stdin, stdout and stderr specify the executed programs' standard
input, standard output and standard error file handles,
respectively. Valid values are PIPE, an existing file
descriptor (a positive integer), an existing file object, and
None. PIPE indicates that a new pipe to the child should be
created. With None, no redirection will occur; the child's file
handles will be inherited from the parent. Additionally, stderr
can be STDOUT, which indicates that the stderr data from the
applications should be captured into the same file handle as for
stdout.

- If preexec_fn is set to a callable object, this object will be
called in the child process just before the child is executed,
with arguments preexec_args.

- If close_fds is true, all file descriptors except 0, 1 and 2
will be closed before the child process is executed.

- If cwd is not None, the current directory will be changed to cwd
before the child is executed.

- If env is not None, it defines the environment variables for the
new process.

- If universal_newlines is true, the file objects fromchild and
childerr are opened as a text files, but lines may be terminated
by any of '\n', the Unix end-of-line convention, '\r', the
Macintosh convention or '\r\n', the Windows convention. All of
these external representations are seen as '\n' by the Python
program. Note: This feature is only available if Python is
built with universal newline support (the default). Also, the
newlines attribute of the file objects fromchild, tochild and
childerr are not updated by the communicate() method.

The module also defines one shortcut function:

run(*args):
Run command with arguments. Wait for command to complete,
then return the returncode attribute. Example:

retcode = popen5.run("stty", "sane")
Exceptions
----------
Exceptions raised in the child process, before the new program has
started to execute, will be re-raised in the parent. Additionally,
the exception object will have one extra attribute called
'child_traceback', which is a string containing traceback
information from the child's point of view.

The most common exception raised is OSError. This occurs, for
example, when trying to execute a non-existent file. Applications
should prepare for OSErrors.

A PopenException will also be raised if Popen is called with
invalid arguments.
Security
--------
popen5 will never call /bin/sh implicitly. This means that all
characters, including shell metacharacters, can safely be passed
to child processes.
Popen objects
-------------
Instances of the Popen class have the following methods:

poll()
Returns -1 if child process hasn't completed yet, or its exit
status otherwise. See below for a description of how the exit
status is encoded.

wait()
Waits for and returns the exit status of the child process.
The exit status encodes both the return code of the process
and information about whether it exited using the exit()
system call or died due to a signal. Functions to help
interpret the status code are defined in the os module (the
W*() family of functions).

communicate(input=None)
Interact with process: Send data to stdin. Read data from
stdout and stderr, until end-of-file is reached. Wait for
process to terminate. The optional stdin argument should be a
string to be sent to the child process, or None, if no data
should be sent to the child.

communicate() returns a tuple (stdout, stderr).

Note: The data read is buffered in memory, so do not use this
method if the data size is large or unlimited.

The following attributes are also available:

fromchild
A file object that provides output from the child process.

tochild
A file object that provides input to the child process.

childerr
A file object that provides error output from the child
process.

pid
The process ID of the child process.

returncode
The child return code. A None value indicates that the
process hasn't terminated yet. A negative value means that
the process was terminated by a signal with number
-returncode.
Open Issues

Perhaps the module should be called something like "process",
instead of "popen5".
Reference Implementation

A reference implementation is available from
http://www.lysator.liu.se/~astrand/popen5/.
References

[1] Secure Programming for Linux and Unix HOWTO, section 8.3.
http://www.dwheeler.com/secure-programs/

[2] Python Dialog
http://pythondialog.sourceforge.net/
Copyright

This document has been placed in the public domain.


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End:
--
/Peter Åstrand <as*****@lysator.liu.se>

Jul 18 '05 #1
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P: n/a
The popen(3) C library function closes pipes that creep
into the child process by virtue of having been open in
the parent through a previous popen() call. That's to
prevent the new child from holding the old pipe open and
delaying end-of-file.

That's an essential practical consideration that needs
to be addressed at least within the context of the popen5
module. "close_fds" doesn't satisfy this requirement,
because 1) it's much broader in its effect, and 2) it's
optional. popen(3) might keep a static module scoped
list of processes that it has created and their pipes,
for this purpose. Maybe popen5 could do the same.

This also makes it dangerous to use different pipe creating
facilities concurrently. Since each can see only the pipes
it creates, a pipe created by popen2 might hang one created
by popen(3) or vice versa (even if popen2 were to address
this problem for its own pipes.) Ideally, all pipe subprocesses
should be created by the same function.

Donn Cave, do**@u.washington.edu
Jul 18 '05 #2

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