Source code: Lib/asyncio/subprocess.py, 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:
import asyncio async def run(cmd): proc = await asyncio.create_subprocess_shell( cmd, stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.PIPE) stdout, stderr = await proc.communicate() print(f'[{cmd!r} exited with {proc.returncode}]') if stdout: print(f'[stdout]\n{stdout.decode()}') if stderr: print(f'[stderr]\n{stderr.decode()}') asyncio.run(run('ls /zzz'))
will print:
['ls /zzz' exited with 1] [stderr] ls: /zzz: No such file or directory
Because 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:
async def main(): await asyncio.gather( run('ls /zzz'), run('sleep 1; echo "hello"')) asyncio.run(main())
See also the Examples subsection.
coroutine asyncio.create_subprocess_exec(program, *args, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)
Create a subprocess.
The limit argument sets the buffer limit for StreamReader
wrappers for Process.stdout
and Process.stderr
(if subprocess.PIPE
is passed to stdout and stderr arguments).
Return a Process
instance.
See the documentation of loop.subprocess_exec()
for other parameters.
Deprecated since version 3.8, will be removed in version 3.10: The loop parameter.
coroutine asyncio.create_subprocess_shell(cmd, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)
Run the cmd shell command.
The limit argument sets the buffer limit for StreamReader
wrappers for Process.stdout
and Process.stderr
(if subprocess.PIPE
is passed to stdout and stderr arguments).
Return a Process
instance.
See the documentation of loop.subprocess_shell()
for other parameters.
Important
It is the application’s responsibility to ensure that all whitespace and special characters are quoted appropriately to avoid shell injection vulnerabilities. The shlex.quote()
function can be used to properly escape whitespace and special shell characters in strings that are going to be used to construct shell commands.
Deprecated since version 3.8, will be removed in version 3.10: The loop parameter.
Note
Subprocesses are available for Windows if a ProactorEventLoop
is used. See Subprocess Support on Windows for details.
See also
asyncio also has the following low-level APIs to work with subprocesses: loop.subprocess_exec()
, loop.subprocess_shell()
, loop.connect_read_pipe()
, loop.connect_write_pipe()
, as well as the Subprocess Transports and Subprocess Protocols.
asyncio.subprocess.PIPE
Can be passed to the stdin, stdout or stderr parameters.
If PIPE is passed to stdin argument, the Process.stdin
attribute will point to a StreamWriter
instance.
If PIPE is passed to stdout or stderr arguments, the Process.stdout
and Process.stderr
attributes will point to StreamReader
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 os.devnull
will be used for the corresponding subprocess stream.
Both create_subprocess_exec()
and create_subprocess_shell()
functions return instances of the Process class. Process is a high-level wrapper that allows communicating with subprocesses and watching for their completion.
class asyncio.subprocess.Process
An object that wraps OS processes created by the create_subprocess_exec()
and create_subprocess_shell()
functions.
This class is designed to have a similar API to the subprocess.Popen
class, but there are some notable differences:
poll()
method;communicate()
and wait()
methods don’t have a timeout parameter: use the wait_for()
function;Process.wait()
method is asynchronous, whereas subprocess.Popen.wait()
method is implemented as a blocking busy loop;This class is not thread safe.
See also the Subprocess and Threads section.
coroutine wait()
Wait for the child process to terminate.
Set and return the returncode
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 communicate()
method when using pipes to avoid this condition.
coroutine communicate(input=None)
Interact with process:
None
);The optional input argument is the data (bytes
object) that will be sent to the child process.
Return a tuple (stdout_data, stderr_data)
.
If either BrokenPipeError
or ConnectionResetError
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.
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 process.
On POSIX systems this method sends signal.SIGTERM
to the child process.
On Windows the Win32 API function TerminateProcess()
is called to stop the child process.
kill()
Kill the child.
On POSIX systems this method sends SIGKILL
to the child process.
On Windows this method is an alias for terminate()
.
stdin
Standard input stream (StreamWriter
) or None
if the process was created with stdin=None
.
stdout
Standard output stream (StreamReader
) or None
if the process was created with stdout=None
.
stderr
Standard error stream (StreamReader
) or None
if the process was created with stderr=None
.
Warning
Use the communicate()
method rather than process.stdin.write()
, await process.stdout.read()
or await process.stderr.read
. 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 create_subprocess_shell()
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.
A negative value -N
indicates that the child was terminated by signal N
(POSIX only).
Standard asyncio event loop supports running subprocesses from different threads by default.
On Windows subprocesses are provided by ProactorEventLoop
only (default), SelectorEventLoop
has no subprocess support.
On UNIX child watchers are used for subprocess finish waiting, see Process Watchers for more info.
Changed in version 3.8: UNIX switched to use ThreadedChildWatcher
for spawning subprocesses from different threads without any limitation.
Spawning a subprocess with inactive current child watcher raises RuntimeError
.
Note that alternative event loop implementations might have own limitations; please refer to their documentation.
See also
The Concurrency and multithreading in asyncio section.
An example using the Process
class to control a subprocess and the StreamReader
class to read from its standard output.
The subprocess is created by the create_subprocess_exec()
function:
import asyncio import sys async def get_date(): code = 'import datetime; print(datetime.datetime.now())' # Create the subprocess; redirect the standard output # into a pipe. proc = await asyncio.create_subprocess_exec( sys.executable, '-c', code, stdout=asyncio.subprocess.PIPE) # Read one line of output. data = await proc.stdout.readline() line = data.decode('ascii').rstrip() # Wait for the subprocess exit. await proc.wait() return line date = asyncio.run(get_date()) print(f"Current date: {date}")
See also the same example written using low-level APIs.
© 2001–2020 Python Software Foundation
Licensed under the PSF License.
https://docs.python.org/3.9/library/asyncio-subprocess.html