18.5.6. Subprocess¶
18.5.6.1. 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
18.5.6.2. 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 aProcess
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 aProcess
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.
18.5.6.3. 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 librarysubprocess.Popen
class called with shell=False and the list of strings passed as the first argument; however, wherePopen
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 constantsubprocess.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 constantsubprocess.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 constantssubprocess.PIPE
(the default) orsubprocess.STDOUT
. By default a new pipe will be created and connected. Whensubprocess.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 ofBaseSubprocessTransport
.This method is a coroutine.
See the constructor of the
subprocess.Popen
class for parameters.- stdin: Either a file-like object representing the pipe to be connected
to the subprocess’s standard input stream using
-
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 withshell=True
.See
subprocess_exec()
for more details about the remaining arguments.Returns a pair of
(transport, protocol)
, where transport is an instance ofBaseSubprocessTransport
.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.
18.5.6.4. Constants¶
-
asyncio.subprocess.
PIPE
¶ Special value that can be used as the stdin, stdout or stderr argument to
create_subprocess_shell()
andcreate_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()
andcreate_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()
andcreate_subprocess_exec()
and indicates that the special fileos.devnull
will be used.
18.5.6.5. Process¶
-
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 signalN
(Unix only).
-
stdin
¶ Standard input stream (write),
None
if the process was created withstdin=None
.
-
stdout
¶ Standard output stream (read),
None
if the process was created withstdout=None
.
-
stderr
¶ Standard error stream (read),
None
if the process was created withstderr=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
orConnectionResetError
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 thanNone
in the result tuple, you need to givestdout=PIPE
and/orstderr=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
andConnectionResetError
.
-
kill
()¶ Kills the child. On Posix OSs the function sends
SIGKILL
to the child. On Windowskill()
is an alias forterminate()
.
-
send_signal
(signal)¶ Sends the signal signal to the child process.
Note
On Windows,
SIGTERM
is an alias forterminate()
.CTRL_C_EVENT
andCTRL_BREAK_EVENT
can be sent to processes started with a creationflags parameter which includesCREATE_NEW_PROCESS_GROUP
.
-
terminate
()¶ Stop the child. On Posix OSs the method sends
signal.SIGTERM
to the child. On Windows the Win32 API functionTerminateProcess()
is called to stop the child.
-
wait():
Wait for child process to terminate. Set and return
returncode
attribute.This method is a coroutine.
-
18.5.6.6. 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()