python_doc_md/Intermediate/library/asyncio-subprocess.md

::: currentmodule asyncio :::

Subprocesses {#asyncio-subprocess}

Source code: Lib/asyncio/subprocess.py{.interpreted-text role="source"}, Lib/asyncio/base_subprocess.py{.interpreted-text role="source"}

---

This section describes high-level async/await asyncio APIs to create and manage subprocesses.

::: {#asyncio_example_subprocess_shell} 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.

Creating Subprocesses

:::: {.function async=""} create_subprocess_exec(program, args, stdin=None, stdout=None, stderr=None, limit=None,*kwds)

Create a subprocess.

The limit argument sets the buffer limit for StreamReader{.interpreted-text role="class"} wrappers for ~asyncio.subprocess.Process.stdout{.interpreted-text role="attr"} and ~asyncio.subprocess.Process.stderr{.interpreted-text role="attr"} (if subprocess.PIPE{.interpreted-text role="const"} is passed to stdout and stderr arguments).

Return a ~asyncio.subprocess.Process{.interpreted-text role="class"} instance.

See the documentation of loop.subprocess_exec{.interpreted-text role="meth"} for other parameters.

If the process object is garbage collected while the process is still running, the child process will be killed.

::: versionchanged 3.10 Removed the loop parameter. ::: ::::

:::::: {.function async=""} create_subprocess_shell(cmd, stdin=None, stdout=None, stderr=None, limit=None, **kwds)

Run the cmd shell command.

The limit argument sets the buffer limit for StreamReader{.interpreted-text role="class"} wrappers for ~asyncio.subprocess.Process.stdout{.interpreted-text role="attr"} and ~asyncio.subprocess.Process.stderr{.interpreted-text role="attr"} (if subprocess.PIPE{.interpreted-text role="const"} is passed to stdout and stderr arguments).

Return a ~asyncio.subprocess.Process{.interpreted-text role="class"} instance.

See the documentation of loop.subprocess_shell{.interpreted-text role="meth"} for other parameters.

If the process object is garbage collected while the process is still running, the child process will be killed.

:::: important ::: title 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{.interpreted-text role="func"} function can be used to properly escape whitespace and special shell characters in strings that are going to be used to construct shell commands. ::::

::: versionchanged 3.10 Removed the loop parameter. ::: ::::::

:::: note ::: title Note :::

Subprocesses are available for Windows if a ProactorEventLoop{.interpreted-text role="class"} is used. See Subprocess Support on Windows <asyncio-windows-subprocess>{.interpreted-text role="ref"} for details. ::::

::: seealso asyncio also has the following low-level APIs to work with subprocesses: loop.subprocess_exec{.interpreted-text role="meth"}, loop.subprocess_shell{.interpreted-text role="meth"}, loop.connect_read_pipe{.interpreted-text role="meth"}, loop.connect_write_pipe{.interpreted-text role="meth"}, as well as the Subprocess Transports <asyncio-subprocess-transports>{.interpreted-text role="ref"} and Subprocess Protocols <asyncio-subprocess-protocols>{.interpreted-text role="ref"}. :::

Constants

::: {.data module=""} asyncio.subprocess.PIPE

Can be passed to the stdin, stdout or stderr parameters.

If PIPE is passed to stdin argument, the Process.stdin <asyncio.subprocess.Process.stdin>{.interpreted-text role="attr"} attribute will point to a ~asyncio.StreamWriter{.interpreted-text role="class"} instance.

If PIPE is passed to stdout or stderr arguments, the Process.stdout <asyncio.subprocess.Process.stdout>{.interpreted-text role="attr"} and Process.stderr <asyncio.subprocess.Process.stderr>{.interpreted-text role="attr"} attributes will point to ~asyncio.StreamReader{.interpreted-text role="class"} instances. :::

::: {.data module=""} asyncio.subprocess.STDOUT

Special value that can be used as the stderr argument and indicates that standard error should be redirected into standard output. :::

::: {.data module=""} 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{.interpreted-text role="data"} will be used for the corresponding subprocess stream. :::

Interacting with Subprocesses

Both create_subprocess_exec{.interpreted-text role="func"} and create_subprocess_shell{.interpreted-text role="func"} functions return instances of the Process class. Process is a high-level wrapper that allows communicating with subprocesses and watching for their completion.

:::::::::::::::::::: {.asyncio.subprocess.Process module=""} An object that wraps OS processes created by the ~asyncio.create_subprocess_exec{.interpreted-text role="func"} and ~asyncio.create_subprocess_shell{.interpreted-text role="func"} functions.

This class is designed to have a similar API to the subprocess.Popen{.interpreted-text role="class"} class, but there are some notable differences:

• unlike Popen, Process instances do not have an equivalent to the ~subprocess.Popen.poll{.interpreted-text role="meth"} method;
• the ~asyncio.subprocess.Process.communicate{.interpreted-text role="meth"} and ~asyncio.subprocess.Process.wait{.interpreted-text role="meth"} methods don't have a timeout parameter: use the ~asyncio.wait_for{.interpreted-text role="func"} function;
• the Process.wait() <asyncio.subprocess.Process.wait>{.interpreted-text role="meth"} method is asynchronous, whereas subprocess.Popen.wait{.interpreted-text role="meth"} method is implemented as a blocking busy loop;
• the universal_newlines parameter is not supported.

This class is not thread safe <asyncio-multithreading>{.interpreted-text role="ref"}.

See also the Subprocess and Threads <asyncio-subprocess-threads>{.interpreted-text role="ref"} section.

::::: {.method async=""} wait()

Wait for the child process to terminate.

Set and return the returncode{.interpreted-text role="attr"} attribute.

:::: note ::: title 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{.interpreted-text role="meth"} method when using pipes to avoid this condition. :::: :::::

:::: {.method async=""} communicate(input=None)

Interact with process:

1. send data to stdin (if input is not None);
2. closes stdin;
3. read data from stdout and stderr, until EOF is reached;
4. wait for process to terminate.

The optional input argument is the data (bytes{.interpreted-text role="class"} object) that will be sent to the child process.

Return a tuple (stdout_data, stderr_data).

If either BrokenPipeError{.interpreted-text role="exc"} or ConnectionResetError{.interpreted-text role="exc"} 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.

::: versionchanged 3.12

stdin gets closed when input=None too. ::: ::::

::::: method send_signal(signal)

Sends the signal signal to the child process.

:::: note ::: title Note :::

On Windows, ~signal.SIGTERM{.interpreted-text role="py:const"} is an alias for terminate{.interpreted-text role="meth"}. CTRL_C_EVENT and CTRL_BREAK_EVENT can be sent to processes started with a creationflags parameter which includes CREATE_NEW_PROCESS_GROUP. :::: :::::

::: method terminate()

Stop the child process.

On POSIX systems this method sends ~signal.SIGTERM{.interpreted-text role="py:const"} to the child process.

On Windows the Win32 API function !TerminateProcess{.interpreted-text role="c:func"} is called to stop the child process. :::

::: method kill()

Kill the child process.

On POSIX systems this method sends ~signal.SIGKILL{.interpreted-text role="py:data"} to the child process.

On Windows this method is an alias for terminate{.interpreted-text role="meth"}. :::

::: attribute stdin

Standard input stream (~asyncio.StreamWriter{.interpreted-text role="class"}) or None if the process was created with stdin=None. :::

::: attribute stdout

Standard output stream (~asyncio.StreamReader{.interpreted-text role="class"}) or None if the process was created with stdout=None. :::

::: attribute stderr

Standard error stream (~asyncio.StreamReader{.interpreted-text role="class"}) or None if the process was created with stderr=None. :::

:::: warning ::: title Warning :::

Use the communicate{.interpreted-text role="meth"} method rather than process.stdin.write() <stdin>{.interpreted-text role="attr"}, await process.stdout.read() <stdout>{.interpreted-text role="attr"} or await process.stderr.read() <stderr>{.interpreted-text role="attr"}. This avoids deadlocks due to streams pausing reading or writing and blocking the child process. ::::

::: attribute pid

Process identification number (PID).

Note that for processes created by the ~asyncio.create_subprocess_shell{.interpreted-text role="func"} function, this attribute is the PID of the spawned shell. :::

::: attribute 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). ::: ::::::::::::::::::::

Subprocess and Threads {#asyncio-subprocess-threads}

Standard asyncio event loop supports running subprocesses from different threads by default.

On Windows subprocesses are provided by ProactorEventLoop{.interpreted-text role="class"} only (default), SelectorEventLoop{.interpreted-text role="class"} has no subprocess support.

Note that alternative event loop implementations might have own limitations; please refer to their documentation.

::: seealso The Concurrency and multithreading in asyncio <asyncio-multithreading>{.interpreted-text role="ref"} section. :::

Examples

An example using the ~asyncio.subprocess.Process{.interpreted-text role="class"} class to control a subprocess and the StreamReader{.interpreted-text role="class"} class to read from its standard output.

::: {#asyncio_example_create_subprocess_exec} The subprocess is created by the create_subprocess_exec{.interpreted-text role="func"} 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 <asyncio_example_subprocess_proto>{.interpreted-text role="ref"} written using low-level APIs.