Source code:
,
Lib/asyncio/base_subprocess.py
———
This section describes high-level async/await asyncio APIs to create and manage subprocesses.
Here’s an example of how asyncio can run a shell command and obtain its result:
importasyncioasyncdefrun(cmd):proc=awaitasyncio.create_subprocess_shell(cmd,stdout=asyncio.subprocess.PIPE,stderr=asyncio.subprocess.PIPE)stdout,stderr=awaitproc.communicate()print(f'[{cmd!r} exited with {proc.returncode}]')ifstdout:print(f'[stdout]\n{stdout.decode()}')ifstderr:print(f'[stderr]\n{stderr.decode()}')asyncio.run(run('ls /zzz'))will print:
['ls /zzz'exitedwith1][stderr]ls:/zzz:NosuchfileordirectoryBecause all asyncio subprocess functions are asynchronous and asyncio provides many tools to work with such functions, it is easy to execute and monitor multiple subprocesses in parallel. It is indeed trivial to modify the above example to run several commands simultaneously:
asyncdefmain():awaitasyncio.gather(run('ls /zzz'),run('sleep 1; echo "hello"'))asyncio.run(main())See also the
subsection.
Creating Subprocesses
asyncasyncio.create_subprocess_exec(program, *args, stdin=None, stdout=None, stderr=None, limit=65536, **kwds)
Create a subprocess.
The limit argument sets the buffer limit for
wrappers for
and
(if
is passed to stdout and stderr arguments).
Return a
instance.
See the documentation of
for other parameters.
If the process object is garbage collected while the process is still running, the child process will be killed.
Changed in version 3.10: Removed the loop parameter.
asyncasyncio.create_subprocess_shell(cmd, stdin=None, stdout=None, stderr=None, limit=65536, **kwds)
Run the cmd shell command.
The limit argument sets the buffer limit for
wrappers for
and
(if
is passed to stdout and stderr arguments).
Return a
instance.
See the documentation of
for other parameters.
If the process object is garbage collected while the process is still running, the child process will be killed.
Important
It is the application’s responsibility to ensure that all whitespace and special characters are quoted appropriately to avoid
vulnerabilities. The
function can be used to properly escape whitespace and special shell characters in strings that are going to be used to construct shell commands.
Changed in version 3.10: Removed the loop parameter.
Constants
asyncio.subprocess.PIPE
Can be passed to the stdin, stdout or stderr parameters.
If PIPE is passed to stdin argument, the
attribute will point to a
instance.
If PIPE is passed to stdout or stderr arguments, the
and
attributes will point to
instances.
asyncio.subprocess.STDOUT
Special value that can be used as the stderr argument and indicates that standard error should be redirected into standard output.
asyncio.subprocess.DEVNULL
Special value that can be used as the stdin, stdout or stderr argument to process creation functions. It indicates that the special file
will be used for the corresponding subprocess stream.
Interacting with Subprocesses
Both
and
functions return instances of the Process class. Process is a high-level wrapper that allows communicating with subprocesses and watching for their completion.
classasyncio.subprocess.Process
An object that wraps OS processes created by the
and
functions.
This class is designed to have a similar API to the
class, but there are some notable differences:
unlike Popen, Process instances do not have an equivalent to the
method;
the
and
methods don’t have a timeout parameter: use the
function;
the
method is asynchronous, whereas
method is implemented as a blocking busy loop;
the universal_newlines parameter is not supported.
This class is
.
See also the
section.
asyncwait()
Wait for the child process to terminate.
Set and return the
attribute.
Note
This method can deadlock when using stdout=PIPE or stderr=PIPE and the child process generates so much output that it blocks waiting for the OS pipe buffer to accept more data. Use the
method when using pipes to avoid this condition.
asynccommunicate(input=None)
Interact with process:
send data to stdin (if input is not None);
closes stdin;
read data from stdout and stderr, until EOF is reached;
wait for process to terminate.
The optional input argument is the data (
object) that will be sent to the child process.
Return a tuple (stdout_data,stderr_data).
If either
or
exception is raised when writing input into stdin, the exception is ignored. This condition occurs when the process exits before all data are written into stdin.
If it is desired to send data to the process’ stdin, the process needs to be created with stdin=PIPE. Similarly, to get anything other than None in the result tuple, the process has to be created with stdout=PIPE and/or stderr=PIPE arguments.
Note, that the data read is buffered in memory, so do not use this method if the data size is large or unlimited.
Changed in version 3.12: stdin gets closed when input=None too.
send_signal(signal)
Sends the signal signal to the child process.
Note
On Windows,
is an alias for
. 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 process.
On POSIX systems this method sends
to the child process.
On Windows the Win32 API function TerminateProcess() is called to stop the child process.
kill()
Kill the child process.
On POSIX systems this method sends
to the child process.
On Windows this method is an alias for
.
stdin
Standard input stream (
) or None if the process was created with stdin=None.
stdout
Standard output stream (
) or None if the process was created with stdout=None.
stderr
Standard error stream (
) or None if the process was created with stderr=None.
Warning
Use the
method rather than
,
or
. This avoids deadlocks due to streams pausing reading or writing and blocking the child process.
pid
Process identification number (PID).
Note that for processes created by the
function, this attribute is the PID of the spawned shell.
returncode
Return code of the process when it exits.
A None value indicates that the process has not terminated yet.
For processes created with
, a negative value -N indicates that the child was terminated by signal N (POSIX only).
For processes created with
, the return code reflects the exit status of the shell itself (e.g. /bin/sh), which may map signals to codes such as 128+N. See the documentation of the shell (for example, the Bash manual’s Exit Status) for details.
Subprocess and Threads
Standard asyncio event loop supports running subprocesses from different threads by default.
On Windows subprocesses are provided by
only (default),
has no subprocess support.
Note that alternative event loop implementations might have own limitations; please refer to their documentation.
Examples
An example using the
class to control a subprocess and the
class to read from its standard output.
The subprocess is created by the
function:
importasyncioimportsysasyncdefget_date():code='import datetime as dt; print(dt.datetime.now())'# Create the subprocess; redirect the standard output# into a pipe.proc=awaitasyncio.create_subprocess_exec(sys.executable,'-c',code,stdout=asyncio.subprocess.PIPE)# Read one line of output.data=awaitproc.stdout.readline()line=data.decode('ascii').rstrip()# Wait for the subprocess exit.awaitproc.wait()returnlinedate=asyncio.run(get_date())print(f"Current date: {date}")See also the
written using low-level APIs.