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.
Hereâs 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.
Creating Subprocesses¶Create a subprocess.
The limit argument sets the buffer limit for StreamReader wrappers for stdout and 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.
Changed in version 3.10: Removed the loop parameter.
Run the cmd shell command.
The limit argument sets the buffer limit for StreamReader wrappers for stdout and 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 applicationâs 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.
Changed in version 3.10: Removed the loop parameter.
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.
Special value that can be used as the stderr argument and indicates that standard error should be redirected into standard output.
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.
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:
unlike Popen, Process instances do not have an equivalent to the poll() method;
the communicate() and wait() methods donât have a timeout parameter: use the wait_for() function;
the Process.wait() method is asynchronous, whereas subprocess.Popen.wait() method is implemented as a blocking busy loop;
the universal_newlines parameter is not supported.
This class is not thread safe.
See also the Subprocess and Threads section.
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.
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 (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 processâ 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.
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.
Stop the child process.
On POSIX systems this method sends SIGTERM to the child process.
On Windows the Win32 API function TerminateProcess() is called to stop the child process.
Kill the child process.
On POSIX systems this method sends SIGKILL to the child process.
On Windows this method is an alias for terminate().
Standard input stream (StreamWriter) or None if the process was created with stdin=None.
Standard output stream (StreamReader) or None if the process was created with stdout=None.
Standard error stream (StreamReader) or None if the process was created with stderr=None.
Process identification number (PID).
Note that for processes created by the create_subprocess_shell() function, this attribute is the PID of the spawned shell.
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.
Examples¶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.
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