diff -r ffa851df0825 -r 2fb8b9db1c86 symbian-qemu-0.9.1-12/python-2.6.1/Doc/library/subprocess.rst --- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/symbian-qemu-0.9.1-12/python-2.6.1/Doc/library/subprocess.rst Fri Jul 31 15:01:17 2009 +0100 @@ -0,0 +1,468 @@ + +:mod:`subprocess` --- Subprocess management +=========================================== + +.. module:: subprocess + :synopsis: Subprocess management. +.. moduleauthor:: Peter Åstrand +.. sectionauthor:: Peter Åstrand + + +.. versionadded:: 2.4 + +The :mod:`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:: + + os.system + os.spawn* + os.popen* + popen2.* + commands.* + +Information about how the :mod:`subprocess` module can be used to replace these +modules and functions can be found in the following sections. + +.. seealso:: + + :pep:`324` -- PEP proposing the subprocess module + + +Using the subprocess Module +--------------------------- + +This module defines one class called :class:`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 the string if a + string is given, but can be explicitly set by using the *executable* + argument. + + On Unix, with *shell=False* (default): In this case, the Popen class uses + :meth:`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 :class:`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 :meth:`list2cmdline` method. Please note that + not all MS Windows applications interpret the command line the same way: + :meth:`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: :const:`0` means unbuffered, :const:`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 :const:`0` (unbuffered). + + The *executable* argument specifies the program to execute. It is very seldom + needed: Usually, the program to execute is defined by the *args* argument. If + ``shell=True``, the *executable* argument specifies which shell to use. On Unix, + the default shell is :file:`/bin/sh`. On Windows, the default shell is + specified by the :envvar:`COMSPEC` environment variable. + + *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. (Unix only) + + If *close_fds* is true, all file descriptors except :const:`0`, :const:`1` and + :const:`2` will be closed before the child process is executed. (Unix only). + Or, on Windows, if *close_fds* is true then no handles will be inherited by the + child process. Note that on Windows, you cannot set *close_fds* to true and + also redirect the standard handles by setting *stdin*, *stdout* or *stderr*. + + If *shell* is :const:`True`, the specified command will be executed through the + shell. + + If *cwd* is not ``None``, the child's current directory will be changed to *cwd* + before it is executed. Note that this directory is not considered when + searching the executable, so you can't specify the program's path relative to + *cwd*. + + If *env* is not ``None``, it must be a mapping that defines the environment + variables for the new process; these are used instead of inheriting the current + process' environment, which is the default behavior. + + If *universal_newlines* is :const:`True`, the file objects stdout and stderr are + opened as text files, but lines may be terminated by any of ``'\n'``, the Unix + end-of-line convention, ``'\r'``, the old 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 :attr:`stdout`, + :attr:`stdin` and :attr:`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) + + +Convenience Functions +^^^^^^^^^^^^^^^^^^^^^ + +This module also defines two shortcut functions: + + +.. function:: call(*popenargs, **kwargs) + + Run command with arguments. Wait for command to complete, then return the + :attr:`returncode` attribute. + + The arguments are the same as for the Popen constructor. Example:: + + retcode = call(["ls", "-l"]) + + +.. function:: check_call(*popenargs, **kwargs) + + Run command with arguments. Wait for command to complete. If the exit code was + zero then return, otherwise raise :exc:`CalledProcessError`. The + :exc:`CalledProcessError` object will have the return code in the + :attr:`returncode` attribute. + + The arguments are the same as for the Popen constructor. Example:: + + check_call(["ls", "-l"]) + + .. versionadded:: 2.5 + + +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 :attr:`child_traceback`, which is a string +containing traceback information from the childs point of view. + +The most common exception raised is :exc:`OSError`. This occurs, for example, +when trying to execute a non-existent file. Applications should prepare for +:exc:`OSError` exceptions. + +A :exc:`ValueError` will be raised if :class:`Popen` is called with invalid +arguments. + +check_call() will raise :exc:`CalledProcessError`, if the called process returns +a non-zero return code. + + +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. + + +Popen Objects +------------- + +Instances of the :class:`Popen` class have the following methods: + + +.. method:: Popen.poll() + + Check if child process has terminated. Set and return :attr:`returncode` + attribute. + + +.. method:: Popen.wait() + + Wait for child process to terminate. Set and return :attr:`returncode` + attribute. + + .. warning:: + + This will deadlock if the child process generates enough output to a + stdout or stderr pipe such that it blocks waiting for the OS pipe buffer + to accept more data. Use :meth:`communicate` to avoid that. + + +.. method:: Popen.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 + *input* argument should be a string to be sent to the child process, or + ``None``, if no data should be sent to the child. + + :meth:`communicate` returns a tuple ``(stdout, stderr)``. + + Note that if you want to send data to the process's stdin, you need to create + the Popen object with ``stdin=PIPE``. Similarly, to get anything other than + ``None`` in the result tuple, you need to give ``stdout=PIPE`` and/or + ``stderr=PIPE`` too. + + .. note:: + + The data read is buffered in memory, so do not use this method if the data + size is large or unlimited. + + +.. method:: Popen.send_signal(signal) + + Sends the signal *signal* to the child. + + .. note:: + + On Windows only SIGTERM is supported so far. It's an alias for + :meth:`terminate`. + + .. versionadded:: 2.6 + + +.. method:: Popen.terminate() + + Stop the child. On Posix OSs the method sends SIGTERM to the + child. On Windows the Win32 API function :cfunc:`TerminateProcess` is called + to stop the child. + + .. versionadded:: 2.6 + + +.. method:: Popen.kill() + + Kills the child. On Posix OSs the function sends SIGKILL to the child. + On Windows :meth:`kill` is an alias for :meth:`terminate`. + + .. versionadded:: 2.6 + + +The following attributes are also available: + +.. warning:: + + Use :meth:`communicate` rather than :meth:`.stdin.write`, + :meth:`.stdout.read` or :meth:`.stderr.read` to avoid deadlocks due + to any of the other OS pipe buffers filling up and blocking the child + process. + + +.. attribute:: Popen.stdin + + If the *stdin* argument is ``PIPE``, this attribute is a file object that + provides input to the child process. Otherwise, it is ``None``. + + +.. attribute:: Popen.stdout + + If the *stdout* argument is ``PIPE``, this attribute is a file object that + provides output from the child process. Otherwise, it is ``None``. + + +.. attribute:: Popen.stderr + + If the *stderr* argument is ``PIPE``, this attribute is file object that + provides error output from the child process. Otherwise, it is ``None``. + + +.. attribute:: Popen.pid + + The process ID of the child process. + + +.. attribute:: Popen.returncode + + The child return code, set by :meth:`poll` and :meth:`wait` (and indirectly + by :meth:`communicate`). 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). + + +.. _subprocess-replacements: + +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 :exc:`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 pipeline +^^^^^^^^^^^^^^^^^^^^^^^^ + +:: + + 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) + +Notes: + +* Calling the program through the shell is usually not required. + +* It's easier to look at the :attr:`returncode` attribute than the exit status. + +A more realistic 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, 'r', bufsize) + ==> + pipe = Popen(cmd, shell=True, bufsize=bufsize, stdout=PIPE).stdout + +:: + + pipe = os.popen(cmd, '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 popen2.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. +