18.5.6. Subprocess¶
18.5.6.1. Windows event loop¶
On Windows, the default event loop is SelectorEventLoop which does not
support subprocesses. ProactorEventLoop should be used instead.
Example to use it on Windows:
import asyncio, sys
if sys.platform == 'win32':
    loop = asyncio.ProactorEventLoop()
    asyncio.set_event_loop(loop)
See also
18.5.6.2. Create a subprocess: high-level API using Process¶
- 
coroutine asyncio.create_subprocess_exec(*args, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)¶
- Create a subprocess. - The limit parameter sets the buffer limit passed to the - StreamReader. See- BaseEventLoop.subprocess_exec()for other parameters.- Return a - Processinstance.- This function is a coroutine. 
- 
coroutine asyncio.create_subprocess_shell(cmd, stdin=None, stdout=None, stderr=None, loop=None, limit=None, **kwds)¶
- Run the shell command cmd. - The limit parameter sets the buffer limit passed to the - StreamReader. See- BaseEventLoop.subprocess_shell()for other parameters.- Return a - Processinstance.- It is the application’s responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid shell injection vulnerabilities. The - shlex.quote()function can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands.- This function is a coroutine. 
Use the BaseEventLoop.connect_read_pipe() and
BaseEventLoop.connect_write_pipe() methods to connect pipes.
18.5.6.3. Create a subprocess: low-level API using subprocess.Popen¶
Run subprocesses asynchronously using the subprocess module.
- 
coroutine BaseEventLoop.subprocess_exec(protocol_factory, *args, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, **kwargs)¶
- Create a subprocess from one or more string arguments (character strings or bytes strings encoded to the filesystem encoding), where the first string specifies the program to execute, and the remaining strings specify the program’s arguments. (Thus, together the string arguments form the - sys.argvvalue of the program, assuming it is a Python script.) This is similar to the standard library- subprocess.Popenclass called with shell=False and the list of strings passed as the first argument; however, where- Popentakes a single argument which is list of strings,- subprocess_exec()takes multiple string arguments.- The protocol_factory must instanciate a subclass of the - asyncio.SubprocessProtocolclass.- Other parameters: - stdin: Either a file-like object representing the pipe to be connected
to the subprocess’s standard input stream using
connect_write_pipe(), or the constantsubprocess.PIPE(the default). By default a new pipe will be created and connected.
- stdout: Either a file-like object representing the pipe to be connected
to the subprocess’s standard output stream using
connect_read_pipe(), or the constantsubprocess.PIPE(the default). By default a new pipe will be created and connected.
- stderr: Either a file-like object representing the pipe to be connected
to the subprocess’s standard error stream using
connect_read_pipe(), or one of the constantssubprocess.PIPE(the default) orsubprocess.STDOUT. By default a new pipe will be created and connected. Whensubprocess.STDOUTis specified, the subprocess’s standard error stream will be connected to the same pipe as the standard output stream.
- All other keyword arguments are passed to subprocess.Popenwithout interpretation, except for bufsize, universal_newlines and shell, which should not be specified at all.
 - Returns a pair of - (transport, protocol), where transport is an instance of- BaseSubprocessTransport.- This method is a coroutine. - See the constructor of the - subprocess.Popenclass for parameters.
- stdin: Either a file-like object representing the pipe to be connected
to the subprocess’s standard input stream using
- 
coroutine BaseEventLoop.subprocess_shell(protocol_factory, cmd, *, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, **kwargs)¶
- Create a subprocess from cmd, which is a character string or a bytes string encoded to the filesystem encoding, using the platform’s “shell” syntax. This is similar to the standard library - subprocess.Popenclass called with- shell=True.- The protocol_factory must instanciate a subclass of the - asyncio.SubprocessProtocolclass.- See - subprocess_exec()for more details about the remaining arguments.- Returns a pair of - (transport, protocol), where transport is an instance of- BaseSubprocessTransport.- It is the application’s responsibility to ensure that all whitespace and metacharacters are quoted appropriately to avoid shell injection vulnerabilities. The - shlex.quote()function can be used to properly escape whitespace and shell metacharacters in strings that are going to be used to construct shell commands.- This method is a coroutine. 
See also
The BaseEventLoop.connect_read_pipe() and
BaseEventLoop.connect_write_pipe() methods.
18.5.6.4. Constants¶
- 
asyncio.subprocess.PIPE¶
- Special value that can be used as the stdin, stdout or stderr argument to - create_subprocess_shell()and- create_subprocess_exec()and indicates that a pipe to the standard stream should be opened.
- 
asyncio.subprocess.STDOUT¶
- Special value that can be used as the stderr argument to - create_subprocess_shell()and- create_subprocess_exec()and indicates that standard error should go into the same handle as standard output.
- 
asyncio.subprocess.DEVNULL¶
- Special value that can be used as the stdin, stdout or stderr argument to - create_subprocess_shell()and- create_subprocess_exec()and indicates that the special file- os.devnullwill be used.
18.5.6.5. Process¶
- 
class asyncio.subprocess.Process¶
- A subprocess created by the - create_subprocess_exec()or the- create_subprocess_shell()function.- The API of the - Processclass was designed to be close to the API of the- subprocess.Popenclass, but there are some differences:- There is no explicit poll()method
- The communicate()andwait()methods don’t take a timeout parameter: use thewait_for()function
- The universal_newlines parameter is not supported (only bytes strings are supported)
- The wait()method of theProcessclass is asynchronous whereas thewait()method of thePopenclass is implemented as a busy loop.
 - This class is not thread safe. See also the Subprocess and threads section. - 
coroutine wait()¶
- Wait for child process to terminate. Set and return - returncodeattribute.- This method is a coroutine. - Note - This will deadlock when using - stdout=PIPEor- stderr=PIPEand the child process generates enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data. Use the- communicate()method when using pipes to avoid that.
 - 
coroutine communicate(input=None)¶
- Interact with process: Send data to stdin. Read data from stdout and stderr, until end-of-file is reached. Wait for process to terminate. The optional input argument should be data to be sent to the child process, or - None, if no data should be sent to the child. The type of input must be bytes.- communicate()returns a tuple- (stdout_data, stderr_data).- If a - BrokenPipeErroror- ConnectionResetErrorexception is raised when writing input into stdin, the exception is ignored. It occurs when the process exits before all data are written into stdin.- Note that if you want to send data to the process’s stdin, you need to create the Process object with - stdin=PIPE. Similarly, to get anything other than- Nonein the result tuple, you need to give- stdout=PIPEand/or- stderr=PIPEtoo.- This method is a coroutine. - Note - The data read is buffered in memory, so do not use this method if the data size is large or unlimited. - Changed in version 3.4.2: The method now ignores - BrokenPipeErrorand- ConnectionResetError.
 - 
send_signal(signal)¶
- Sends the signal signal to the child process. - Note - On Windows, - SIGTERMis an alias for- terminate().- CTRL_C_EVENTand- CTRL_BREAK_EVENTcan be sent to processes started with a creationflags parameter which includes- CREATE_NEW_PROCESS_GROUP.
 - 
terminate()¶
- Stop the child. On Posix OSs the method sends - signal.SIGTERMto the child. On Windows the Win32 API function- TerminateProcess()is called to stop the child.
 - 
kill()¶
- Kills the child. On Posix OSs the function sends - SIGKILLto the child. On Windows- kill()is an alias for- terminate().
 - 
stdin¶
- Standard input stream ( - StreamWriter),- Noneif the process was created with- stdin=None.
 - 
stdout¶
- Standard output stream ( - StreamReader),- Noneif the process was created with- stdout=None.
 - 
stderr¶
- Standard error stream ( - StreamReader),- Noneif the process was created with- stderr=None.
 - Warning - Use the - communicate()method rather than- .stdin.write,- .stdout.reador- .stderr.readto avoid deadlocks due to streams pausing reading or writing and blocking the child process.- 
pid¶
- The identifier of the process. - Note that for processes created by the - create_subprocess_shell()function, this attribute is the process identifier of the spawned shell.
 - 
returncode¶
- Return code of the process when it exited. A - Nonevalue indicates that the process has not terminated yet.- A negative value - -Nindicates that the child was terminated by signal- N(Unix only).
 
- There is no explicit 
18.5.6.6. Subprocess and threads¶
asyncio supports running subprocesses from different threads, but there are limits:
- An event loop must run in the main thread
- The child watcher must be instantiated in the main thread, before executing
subprocesses from other threads. Call the get_child_watcher()function in the main thread to instantiate the child watcher.
The asyncio.subprocess.Process class is not thread safe.
See also
The Concurrency and multithreading in asyncio section.
18.5.6.7. Subprocess examples¶
18.5.6.7.1. Subprocess using transport and protocol¶
Example of a subprocess protocol using to get the output of a subprocess and to
wait for the subprocess exit. The subprocess is created by the
BaseEventLoop.subprocess_exec() method:
import asyncio
import sys
class DateProtocol(asyncio.SubprocessProtocol):
    def __init__(self, exit_future):
        self.exit_future = exit_future
        self.output = bytearray()
    def pipe_data_received(self, fd, data):
        self.output.extend(data)
    def process_exited(self):
        self.exit_future.set_result(True)
@asyncio.coroutine
def get_date(loop):
    code = 'import datetime; print(datetime.datetime.now())'
    exit_future = asyncio.Future(loop=loop)
    # Create the subprocess controlled by the protocol DateProtocol,
    # redirect the standard output into a pipe
    create = loop.subprocess_exec(lambda: DateProtocol(exit_future),
                                  sys.executable, '-c', code,
                                  stdin=None, stderr=None)
    transport, protocol = yield from create
    # Wait for the subprocess exit using the process_exited() method
    # of the protocol
    yield from exit_future
    # Close the stdout pipe
    transport.close()
    # Read the output which was collected by the pipe_data_received()
    # method of the protocol
    data = bytes(protocol.output)
    return data.decode('ascii').rstrip()
if sys.platform == "win32":
    loop = asyncio.ProactorEventLoop()
    asyncio.set_event_loop(loop)
else:
    loop = asyncio.get_event_loop()
date = loop.run_until_complete(get_date(loop))
print("Current date: %s" % date)
loop.close()
18.5.6.7.2. Subprocess using streams¶
Example using the Process class to control the
subprocess and the StreamReader class to read from the standard
output.  The subprocess is created by the create_subprocess_exec()
function:
import asyncio.subprocess
import sys
@asyncio.coroutine
def get_date():
    code = 'import datetime; print(datetime.datetime.now())'
    # Create the subprocess, redirect the standard output into a pipe
    create = asyncio.create_subprocess_exec(sys.executable, '-c', code,
                                            stdout=asyncio.subprocess.PIPE)
    proc = yield from create
    # Read one line of output
    data = yield from proc.stdout.readline()
    line = data.decode('ascii').rstrip()
    # Wait for the subprocess exit
    yield from proc.wait()
    return line
if sys.platform == "win32":
    loop = asyncio.ProactorEventLoop()
    asyncio.set_event_loop(loop)
else:
    loop = asyncio.get_event_loop()
date = loop.run_until_complete(get_date())
print("Current date: %s" % date)
loop.close()
