Creating Subprocesses¶

async asyncio.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 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.

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.

async asyncio.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 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.

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 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.

Constants¶

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.

Interacting with Subprocesses¶

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:

• unlike Popen, Process instances do not have an equivalent to the poll() method;

• the communicate() and wait() methods don’t 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.

async 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.

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 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.

If this coroutine is cancelled (for example, when a timeout is set with wait_for()), the output that was already read is not lost: call communicate() again to read the remaining output and get the complete data:

try:
    stdout, stderr = await asyncio.wait_for(
        proc.communicate(), timeout=5.0)
except TimeoutError:
    proc.kill()
    stdout, stderr = await proc.communicate()

Passing input after a previous communicate() call was cancelled raises ValueError; pass input=None to continue the communication, the original input is used.

Changed in version 3.12: stdin gets closed when input=None too.

Changed in version 3.16: If communicate() is cancelled, the output that was already read is now preserved and returned by a subsequent communicate() call. Passing input to a communicate() call following a cancelled one now raises ValueError.

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 SIGTERM 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 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.

For processes created with create_subprocess_exec(), a negative value -N indicates that the child was terminated by signal N (POSIX only).

For processes created with create_subprocess_shell(), 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.

import asyncio
import sys

async def get_date():
    code = 'import datetime as dt; print(dt.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}")