
Subprocess
**********


Windows event loop
==================

On Windows, the default event loop is "SelectorEventLoop" which does
not support subprocesses. "ProactorEventLoop" should be used instead.
Example to use it on Windows:

   import asyncio, os

   if os.name == 'nt':
       loop = asyncio.ProactorEventLoop()
       asyncio.set_event_loop(loop)

See also: *Available event loops* and *Platform support*.


Create a subprocess: high-level API using Process
=================================================

asyncio.create_subprocess_shell(cmd, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)

   Run the shell command *cmd*. See "BaseEventLoop.subprocess_shell()"
   for parameters. Return a "Process" instance.

   The optional *limit* parameter sets the buffer limit passed to the
   "StreamReader".

   This function is a *coroutine*.

asyncio.create_subprocess_exec(*args, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)

   Create a subprocess. See "BaseEventLoop.subprocess_exec()" for
   parameters. Return a "Process" instance.

   The optional *limit* parameter sets the buffer limit passed to the
   "StreamReader".

   This function is a *coroutine*.

Use the "BaseEventLoop.connect_read_pipe()" and
"BaseEventLoop.connect_write_pipe()" methods to connect pipes.


Create a subprocess: low-level API using subprocess.Popen
=========================================================

Run subprocesses asynchronously using the "subprocess" module.

BaseEventLoop.subprocess_exec(protocol_factory, *args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, **kwargs)

   Create a subprocess from one or more string arguments (character
   strings or bytes strings encoded to the *filesystem encoding*),
   where the first string specifies the program to execute, and the
   remaining strings specify the program's arguments. (Thus, together
   the string arguments form the "sys.argv" value of the program,
   assuming it is a Python script.) This is similar to the standard
   library "subprocess.Popen" class called with shell=False and the
   list of strings passed as the first argument; however, where
   "Popen" takes a single argument which is list of strings,
   "subprocess_exec()" takes multiple string arguments.

   Other parameters:

   * *stdin*: Either a file-like object representing the pipe to be
     connected to the subprocess's standard input stream using
     "connect_write_pipe()", or the constant "subprocess.PIPE" (the
     default). By default a new pipe will be created and connected.

   * *stdout*: Either a file-like object representing the pipe to be
     connected to the subprocess's standard output stream using
     "connect_read_pipe()", or the constant "subprocess.PIPE" (the
     default). By default a new pipe will be created and connected.

   * *stderr*: Either a file-like object representing the pipe to be
     connected to the subprocess's standard error stream using
     "connect_read_pipe()", or one of the constants "subprocess.PIPE"
     (the default) or "subprocess.STDOUT". By default a new pipe will
     be created and connected. When "subprocess.STDOUT" is specified,
     the subprocess's standard error stream will be connected to the
     same pipe as the standard output stream.

   * All other keyword arguments are passed to "subprocess.Popen"
     without interpretation, except for *bufsize*,
     *universal_newlines* and *shell*, which should not be specified
     at all.

   Returns a pair of "(transport, protocol)", where *transport* is an
   instance of "BaseSubprocessTransport".

   This method is a *coroutine*.

   See the constructor of the "subprocess.Popen" class for parameters.

BaseEventLoop.subprocess_shell(protocol_factory, cmd, *, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, **kwargs)

   Create a subprocess from *cmd*, which is a character string or a
   bytes string encoded to the *filesystem encoding*, using the
   platform's "shell" syntax. This is similar to the standard library
   "subprocess.Popen" class called with "shell=True".

   See "subprocess_exec()" for more details about the remaining
   arguments.

   Returns a pair of "(transport, protocol)", where *transport* is an
   instance of "BaseSubprocessTransport".

   This method is a *coroutine*.

   See the constructor of the "subprocess.Popen" class for parameters.

See also: The "BaseEventLoop.connect_read_pipe()" and
  "BaseEventLoop.connect_write_pipe()" methods.


Constants
=========

asyncio.subprocess.PIPE

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to "create_subprocess_shell()" and
   "create_subprocess_exec()" and indicates that a pipe to the
   standard stream should be opened.

asyncio.subprocess.STDOUT

   Special value that can be used as the *stderr* argument to
   "create_subprocess_shell()" and "create_subprocess_exec()" and
   indicates that standard error should go into the same handle as
   standard output.

asyncio.subprocess.DEVNULL

   Special value that can be used as the *stdin*, *stdout* or *stderr*
   argument to "create_subprocess_shell()" and
   "create_subprocess_exec()" and indicates that the special file
   "os.devnull" will be used.


Process
=======

class class asyncio.subprocess.Process

   pid

      The identifier of the process.

      Note that if you set the *shell* argument to "True", this is the
      process identifier of the spawned shell.

   returncode

      Return code of the process when it exited.  A "None" value
      indicates that the process has not terminated yet.

      A negative value "-N" indicates that the child was terminated by
      signal "N" (Unix only).

   stdin

      Standard input stream (write), "None" if the process was created
      with "stdin=None".

   stdout

      Standard output stream (read), "None" if the process was created
      with "stdout=None".

   stderr

      Standard error stream (read), "None" if the process was created
      with "stderr=None".

   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
      data to be sent to the child process, or "None", if no data
      should be sent to the child.  The type of *input* must be bytes.

      If a "BrokenPipeError" or "ConnectionResetError" exception is
      raised when writing *input* into stdin, the exception is
      ignored. It occurs when the process exits before all data are
      written into stdin.

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

      Note that if you want to send data to the process's stdin, you
      need to create the Process 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.

      This method is a *coroutine*.

      Changed in version 3.4.2: The method now ignores
      "BrokenPipeError" and "ConnectionResetError".

   kill()

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

   send_signal(signal)

      Sends the signal *signal* to the child process.

      Note: On Windows, "SIGTERM" is an alias for "terminate()".
        "CTRL_C_EVENT" and "CTRL_BREAK_EVENT" can be sent to processes
        started with a *creationflags* parameter which includes
        "CREATE_NEW_PROCESS_GROUP".

   terminate()

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

   wait():

      Wait for child process to terminate.  Set and return
      "returncode" attribute.

      This method is a *coroutine*.


Example
=======

Implement a function similar to "subprocess.getstatusoutput()", except
that it does not use a shell. Get the output of the "python -m
platform" command and display the output:

   import asyncio
   import os
   import sys
   from asyncio import subprocess

   @asyncio.coroutine
   def getstatusoutput(*args):
       proc = yield from asyncio.create_subprocess_exec(
                                     *args,
                                     stdout=subprocess.PIPE,
                                     stderr=subprocess.STDOUT)
       try:
           stdout, _ = yield from proc.communicate()
       except:
           proc.kill()
           yield from proc.wait()
           raise
       exitcode = yield from proc.wait()
       return (exitcode, stdout)

   if os.name == 'nt':
       loop = asyncio.ProactorEventLoop()
       asyncio.set_event_loop(loop)
   else:
       loop = asyncio.get_event_loop()
   coro = getstatusoutput(sys.executable, '-m', 'platform')
   exitcode, stdout = loop.run_until_complete(coro)
   if not exitcode:
       stdout = stdout.decode('ascii').rstrip()
       print("Platform: %s" % stdout)
   else:
       print("Python failed with exit code %s:" % exitcode, flush=True)
       sys.stdout.buffer.write(stdout)
       sys.stdout.buffer.flush()
   loop.close()
