Update of /cvsroot/python/python/dist/src/Doc/lib In directory sc8-pr-cvs1.sourceforge.net:/tmp/cvs-serv1126/lib Modified Files: lib.tex Added Files: libsubprocess.tex Log Message: SF Patch #1048341: subprocess documentation, based on PEP/docstring by Peter Astrand, with markup by Fredrik Lundh and Raymond Hettinger. --- NEW FILE: libsubprocess.tex --- \section{\module{subprocess} --- Subprocess management} \declaremodule{standard}{subprocess} \modulesynopsis{Subprocess management.} \moduleauthor{Peter \AA strand}{astrand@lysator.liu.se} \sectionauthor{Peter \AA strand}{astrand@lysator.liu.se} \versionadded{2.4} The \module{subprocess} module allows you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. This module intends to replace several other, older modules and functions, such as: % XXX Should add pointers to this module to at least the popen2 % and commands sections. \begin{verbatim} os.system os.spawn* os.popen* popen2.* commands.* \end{verbatim} Information about how the \module{subprocess} module can be used to replace these modules and functions can be found in the following sections. \subsection{Using the subprocess Module} This module defines one class called \class{Popen}: \begin{classdesc}{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: \var{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 \var{shell=False} (default): In this case, the Popen class uses \method{os.execvp()} to execute the child program. \var{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 \var{shell=True}: If args is a string, it specifies the command string to execute through the shell. If \var{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 \class{Popen} class uses CreateProcess() to execute the child program, which operates on strings. If \var{args} is a sequence, it will be converted to a string using the \method{list2cmdline} method. Please note that not all MS Windows applications interpret the command line the same way: \method{list2cmdline} is designed for applications using the same rules as the MS C runtime. \var{bufsize}, if given, has the same meaning as the corresponding argument to the built-in open() function: \constant{0} means unbuffered, \constant{1} means line buffered, any other positive value means use a buffer of (approximately) that size. A negative \var{bufsize} means to use the system default, which usually means fully buffered. The default value for \var{bufsize} is \constant{0} (unbuffered). \var{stdin}, \var{stdout} and \var{stderr} specify the executed programs' standard input, standard output and standard error file handles, respectively. Valid values are \code{PIPE}, an existing file descriptor (a positive integer), an existing file object, and \code{None}. \code{PIPE} indicates that a new pipe to the child should be created. With \code{None}, no redirection will occur; the child's file handles will be inherited from the parent. Additionally, \var{stderr} can be \code{STDOUT}, which indicates that the stderr data from the applications should be captured into the same file handle as for stdout. If \var{preexec_fn} is set to a callable object, this object will be called in the child process just before the child is executed. If \var{close_fds} is true, all file descriptors except \constant{0}, \constant{1} and \constant{2} will be closed before the child process is executed. If \var{shell} is \constant{True}, the specified command will be executed through the shell. If \var{cwd} is not \code{None}, the current directory will be changed to cwd before the child is executed. If \var{env} is not \code{None}, it defines the environment variables for the new process. If \var{universal_newlines} is \constant{True}, the file objects stdout and stderr are opened as a text files, but lines may be terminated by any of \code{'\e n'}, the Unix end-of-line convention, \code{'\e r'}, the Macintosh convention or \code{'\e r\e n'}, the Windows convention. All of these external representations are seen as \code{'\e 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 \member{stdout}, \member{stdin} and \member{stderr} are not updated by the communicate() method.} The \var{startupinfo} and \var{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) \end{classdesc} \subsubsection{Convenience Functions} This module also defines one shortcut function: \begin{funcdesc}{call}{*args, **kwargs} Run command with arguments. Wait for command to complete, then return the \member{returncode} attribute. The arguments are the same as for the Popen constructor. Example: \begin{verbatim} retcode = call(["ls", "-l"]) \end{verbatim} \end{funcdesc} \subsubsection{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 \member{child_traceback}, which is a string containing traceback information from the childs point of view. The most common exception raised is \exception{OSError}. This occurs, for example, when trying to execute a non-existent file. Applications should prepare for \exception{OSError} exceptions. A \exception{ValueError} will be raised if \class{Popen} is called with invalid arguments. \subsubsection{Security} Unlike some other popen functions, this implementation will never call /bin/sh implicitly. This means that all characters, including shell metacharacters, can safely be passed to child processes. \subsection{Popen Objects} Instances of the \class{Popen} class have the following methods: \begin{methoddesc}{poll}{} Check if child process has terminated. Returns returncode attribute. \end{methoddesc} \begin{methoddesc}{wait}{} Wait for child process to terminate. Returns returncode attribute. \end{methoddesc} \begin{methoddesc}{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 \var{stdin} argument should be a string to be sent to the child process, or \code{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.} \end{methoddesc} The following attributes are also available: \begin{memberdesc}{stdin} If the \var{stdin} argument is \code{PIPE}, this attribute is a file object that provides input to the child process. Otherwise, it is \code{None}. \end{memberdesc} \begin{memberdesc}{stdout} If the \var{stdout} argument is \code{PIPE}, this attribute is a file object that provides output from the child process. Otherwise, it is \code{None}. \end{memberdesc} \begin{memberdesc}{stderr} If the \var{stderr} argument is \code{PIPE}, this attribute is file object that provides error output from the child process. Otherwise, it is \code{None}. \end{memberdesc} \begin{memberdesc}{pid} The process ID of the child process. \end{memberdesc} \begin{memberdesc}{returncode} The child return code. A \code{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). \end{memberdesc} \subsection{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 \exception{OSError} exception.} In the following examples, we assume that the subprocess module is imported with "from subprocess import *". \subsubsection{Replacing /bin/sh shell backquote} \begin{verbatim} output=`mycmd myarg` ==> output = Popen(["mycmd", "myarg"], stdout=PIPE).communicate()[0] \end{verbatim} \subsubsection{Replacing shell pipe line} \begin{verbatim} output=`dmesg | grep hda` ==> p1 = Popen(["dmesg"], stdout=PIPE) p2 = Popen(["grep", "hda"], stdin=p1.stdout) output = p2.communicate()[0] \end{verbatim} \subsubsection{Replacing os.system()} \begin{verbatim} sts = os.system("mycmd" + " myarg") ==> p = Popen("mycmd" + " myarg", shell=True) sts = os.waitpid(p.pid, 0) \end{verbatim} Notes: \begin{itemize} \item Calling the program through the shell is usually not required. \item It's easier to look at the \member{returncode} attribute than the exit status. \end{itemize} A more realistic example would look like this: \begin{verbatim} 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 \end{verbatim} \subsubsection{Replacing os.spawn*} P_NOWAIT example: \begin{verbatim} pid = os.spawnlp(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg") ==> pid = Popen(["/bin/mycmd", "myarg"]).pid \end{verbatim} P_WAIT example: \begin{verbatim} retcode = os.spawnlp(os.P_WAIT, "/bin/mycmd", "mycmd", "myarg") ==> retcode = call(["/bin/mycmd", "myarg"]) \end{verbatim} Vector example: \begin{verbatim} os.spawnvp(os.P_NOWAIT, path, args) ==> Popen([path] + args[1:]) \end{verbatim} Environment example: \begin{verbatim} os.spawnlpe(os.P_NOWAIT, "/bin/mycmd", "mycmd", "myarg", env) ==> Popen(["/bin/mycmd", "myarg"], env={"PATH": "/usr/bin"}) \end{verbatim} \subsubsection{Replacing os.popen*} \begin{verbatim} pipe = os.popen(cmd, mode='r', bufsize) ==> pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout \end{verbatim} \begin{verbatim} pipe = os.popen(cmd, mode='w', bufsize) ==> pipe = Popen(cmd, shell=True, bufsize=bufsize, stdin=PIPE).stdin \end{verbatim} \begin{verbatim} (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) \end{verbatim} \begin{verbatim} (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) \end{verbatim} \begin{verbatim} (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) \end{verbatim} \subsubsection{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.} \begin{verbatim} (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) \end{verbatim} \begin{verbatim} (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) \end{verbatim} The popen2.Popen3 and popen3.Popen4 basically works as subprocess.Popen, except that: \begin{itemize} \item subprocess.Popen raises an exception if the execution fails \item the \var{capturestderr} argument is replaced with the \var{stderr} argument. \item stdin=PIPE and stdout=PIPE must be specified. \item popen2 closes all file descriptors by default, but you have to specify close_fds=True with subprocess.Popen. \end{itemize} Index: lib.tex =================================================================== RCS file: /cvsroot/python/python/dist/src/Doc/lib/lib.tex,v retrieving revision 1.230 retrieving revision 1.231 diff -u -d -r1.230 -r1.231 --- lib.tex 31 Aug 2004 13:19:43 -0000 1.230 +++ lib.tex 17 Oct 2004 16:29:48 -0000 1.231 @@ -145,6 +145,7 @@ \input{libstatcache} \input{libstatvfs} \input{libfilecmp} +\input{libsubprocess} \input{libpopen2} \input{libdatetime} \input{libtime}