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PEP 324 -- subprocess - New process module

PEP:324
Title:subprocess - New process module
Author:Peter Astrand <astrand at lysator.liu.se>
Status:Final
Type:Standards Track
Created:19-Nov-2003
Python-Version:2.4
Post-History:

Abstract

This PEP describes a new module for starting and communicating with processes.

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 subprocess module 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 the subprocess module, 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:

  • subprocess 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. subprocess, however, contains one single class. Since the subprocess module 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: subprocess.call(). 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 an argument list.
    • The return value is easier to work with.

    The call() utility function accepts an 'args' argument, just like the Popen class constructor. It waits for the command to complete, then returns the returncode attribute. The implementation is very simple:

    def call(*args, **kwargs):
        return Popen(*args, **kwargs).wait()
    

    The motivation behind the call() function is simple: Starting a process and wait for it to finish is a common task.

    While Popen supports a wide range of options, many users have simple needs. Many people are using os.system() today, mainly because it provides a simple interface. Consider this example:

    os.system("stty sane -F " + device)
    

    With subprocess.call(), this would look like:

    subprocess.call(["stty", "sane", "-F", device])
    

    or, if executing through the shell:

    subprocess.call("stty sane -F " + device, shell=True)
    
  • 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.
  • On POSIX platforms, no extension module is required: the module uses os.fork(), os.execvp() etc.

  • On Windows platforms, the module requires either Mark Hammond's Windows extensions [5], or a small extension module called _subprocess.

Specification

This module defines one class called Popen:

class Popen(args, bufsize=0, executable=None,
            stdin=None, stdout=None, stderr=None,
            preexec_fn=None, close_fds=False, shell=False,
            cwd=None, env=None, universal_newlines=False,
            startupinfo=None, creationflags=0):

Arguments are:

  • args should be a string, or a sequence of program arguments. The program to execute is normally the first item in the args sequence or string, but can be explicitly set by using the executable argument.

    On UNIX, with shell=False (default): In this case, the Popen class uses os.execvp() to execute the child program. args should normally be a sequence. A string will be treated as a sequence with the string as the only item (the program to execute).

    On UNIX, with shell=True: If args is a string, it specifies the command string to execute through the shell. If args is a sequence, the first item specifies the command string, and any additional items will be treated as additional shell arguments.

    On Windows: the Popen class uses CreateProcess() to execute the child program, which operates on strings. If args is a sequence, it will be converted to a string using the list2cmdline method. Please note that not all MS Windows applications interpret the command line the same way: The list2cmdline is designed for applications using the same rules as the MS C runtime.

  • 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.

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

  • If shell is true, the specified command will be executed through the shell.

  • 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 stdout and stderr are opened as a text file, 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 stdout, stdin and stderr are not updated by the communicate() method.

  • The startupinfo and creationflags, if given, will be passed to the underlying CreateProcess() function. They can specify things such as appearance of the main window and priority for the new process. (Windows only)

This module also defines two shortcut functions:

  • call(*args, **kwargs):

    Run command with arguments. Wait for command to complete, then return the returncode attribute.

    The arguments are the same as for the Popen constructor. Example:

    retcode = call(["ls", "-l"])
    

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 ValueError will be raised if Popen is called with invalid arguments.

Security

Unlike some other popen functions, this implementation will never call /bin/sh implicitly. This means that all characters, including shell meta-characters, can safely be passed to child processes.

Popen objects

Instances of the Popen class have the following methods:

poll()
Check if child process has terminated. Returns returncode attribute.
wait()
Wait for child process to terminate. Returns returncode attribute.
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:

stdin
If the stdin argument is PIPE, this attribute is a file object that provides input to the child process. Otherwise, it is None.
stdout
If the stdout argument is PIPE, this attribute is a file object that provides output from the child process. Otherwise, it is None.
stderr
If the stderr argument is PIPE, this attribute is file object that provides error output from the child process. Otherwise, it is None.
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 -N indicates that the child was terminated by signal N (UNIX only).

Replacing older functions with the subprocess module

In this section, "a ==> b" means that b can be used as a replacement for a.

Note: All functions in this section fail (more or less) silently if the executed program cannot be found; this module raises an OSError exception.

In the following examples, we assume that the subprocess module is imported with from subprocess import *.

Replacing /bin/sh shell backquote

output=`mycmd myarg`
==>
output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0]

Replacing shell pipe line

output=`dmesg | grep hda`
==>
p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]

Replacing os.system()

sts = os.system("mycmd" + " myarg")
==>
p = Popen("mycmd" + " myarg", shell=True)
sts = os.waitpid(p.pid, 0)

Note:

  • Calling the program through the shell is usually not required.
  • It's easier to look at the returncode attribute than the exit status.

A more real-world example would look like this:

try:
    retcode = call("mycmd" + " myarg", shell=True)
    if retcode < 0:
        print >>sys.stderr, "Child was terminated by signal", -retcode
    else:
        print >>sys.stderr, "Child returned", retcode
except OSError, e:
    print >>sys.stderr, "Execution failed:", e

Replacing os.spawn*

P_NOWAIT example:

pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg")
==>
pid = Popen(["/bin/mycmd", "myarg"]).pid

P_WAIT example:

retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg")
==>
retcode = call(["/bin/mycmd", "myarg"])

Vector example:

os.spawnvp(os.P_NOWAIT, path, args)
==>
Popen([path] + args[1:])

Environment example:

os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env)
==>
Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"})

Replacing os.popen*

pipe = os.popen(cmd, mode='r', bufsize)
==>
pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout

pipe = os.popen(cmd, mode='w', bufsize)
==>
pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin


(child_stdin, child_stdout) = os.popen2(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdin, child_stdout) = (p.stdin, p.stdout)


(child_stdin,
 child_stdout,
 child_stderr) = os.popen3(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, stderr=PIPE, close_fds=True)
(child_stdin,
 child_stdout,
 child_stderr) = (p.stdin, p.stdout, p.stderr)


(child_stdin, child_stdout_and_stderr) = os.popen4(cmd, mode, bufsize)
==>
p = Popen(cmd, shell=True, bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, stderr=STDOUT, close_fds=True)
(child_stdin, child_stdout_and_stderr) = (p.stdin, p.stdout)

Replacing popen2.*

Note: If the cmd argument to popen2 functions is a string, the command is executed through /bin/sh. If it is a list, the command is directly executed.

(child_stdout, child_stdin) = popen2.popen2("somestring", bufsize, mode)
==>
p = Popen(["somestring"], shell=True, bufsize=bufsize
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)


(child_stdout, child_stdin) = popen2.popen2(["mycmd", "myarg"], bufsize, mode)
==>
p = Popen(["mycmd", "myarg"], bufsize=bufsize,
          stdin=PIPE, stdout=PIPE, close_fds=True)
(child_stdout, child_stdin) = (p.stdout, p.stdin)

The popen2.Popen3 and popen3.Popen4 basically works as subprocess.Popen, except that:

  • subprocess.Popen raises an exception if the execution fails
  • the capturestderr argument is replaced with the stderr argument.
  • stdin=PIPE and stdout=PIPE must be specified.
  • popen2 closes all file descriptors by default, but you have to specify close_fds=True with subprocess.Popen.

Open Issues

Some features have been requested but is not yet implemented. This includes:

  • Support for managing a whole flock of subprocesses
  • Support for managing "daemon" processes
  • Built-in method for killing subprocesses

While these are useful features, it's expected that these can be added later without problems.

  • expect-like functionality, including pty support.

pty support is highly platform-dependent, which is a problem. Also, there are already other modules that provide this kind of functionality [6].

Backwards Compatibility

Since this is a new module, no major backward compatible issues are expected. The module name "subprocess" might collide with other, previous modules [3] with the same name, but the name "subprocess" seems to be the best suggested name so far. The first name of this module was "popen5", but this name was considered too unintuitive. For a while, the module was called "process", but this name is already used by Trent Mick's module [4].

The functions and modules that this new module is trying to replace (os.system, os.spawn*, os.popen*, popen2.*, commands.*) are expected to be available in future Python versions for a long time, to preserve backwards compatibility.

Reference Implementation

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

Source: https://github.com/python/peps/blob/master/pep-0324.txt