Subprocess management

Subprocess management

New in version 2.4.

The 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 subprocess module can be used to replace these modules and functions can be found in the following sections.

See also

PEP 324 – PEP proposing the subprocess module
Using the subprocess Module¶

This module defines one class called Popen:

class subprocess.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 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: 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).

    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 /bin/sh. On Windows, the default shell is specified by the 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 0, 1 and 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 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 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 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)

subprocess.PIPE
    Special value that can be used as the stdin, stdout or stderr argument to Popen and indicates that a pipe to the standard stream should be opened.

subprocess.STDOUT
    Special value that can be used as the stderr argument to Popen and indicates that standard error should go into the same handle as standard output.

Convenience Functions

This module also defines two shortcut functions:

subprocess.call(*popenargs, **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"])

subprocess.check_call(*popenargs, **kwargs)

    Run command with arguments. Wait for command to complete. If the exit code was zero then return, otherwise raise CalledProcessError. The CalledProcessError object will have the return code in the returncode attribute.

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

    check_call(["ls", "-l"])

    New in version 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 child_traceback, which is a string containing traceback information from the childs 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 OSError exceptions.

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

check_call() will raise 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 Popen class have the following methods:

Popen.poll()
    Check if child process has terminated. Set and return returncode attribute.

Popen.wait()

    Wait for child process to terminate. Set and return 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 communicate() to avoid that.

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.

    communicate() returns a tuple (stdoutdata, stderrdata).

    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.

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 terminate().

    New in version 2.6.

Popen.terminate()

    Stop the child. On Posix OSs the method sends SIGTERM to the child. On Windows the Win32 API function TerminateProcess is called to stop the child.

    New in version 2.6.

Popen.kill()

    Kills the child. On Posix OSs the function sends SIGKILL to the child. On Windows kill() is an alias for terminate().

    New in version 2.6.

The following attributes are also available:

Warning

Use communicate() rather than stdin.write(), stdout.read() or stderr.read() to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

Popen.stdin
    If the stdin argument was PIPE, this attribute is a file object that provides input to the child process. Otherwise, it is None.

Popen.stdout
    If the stdout argument was PIPE, this attribute is a file object that provides output from the child process. Otherwise, it is None.

Popen.stderr
    If the stderr argument was PIPE, this attribute is a file object that provides error output from the child process. Otherwise, it is None.

Popen.pid
    The process ID of the child process.

Popen.returncode

    The child return code, set by poll() and wait() (and indirectly by 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).

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 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 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 the os.spawn family

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, os.popen2, os.popen3

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 functions from the popen2 module

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)

popen2.Popen3 and popen2.Popen4 basically work as subprocess.Popen, except that:

    * 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 Popen.