PEP: 324 Title: popen5 - New POSIX process module Version: $Revision$ Last-Modified: $Date$ Author: Peter Astrand 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. Local Variables: mode: indented-text indent-tabs-mode: nil sentence-end-double-space: t fill-column: 70 End: