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
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 AbstractEventLoop.subprocess_exec()
for other parameters.
Return a Process
instance.
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 AbstractEventLoop.subprocess_shell()
for other parameters.
Return a Process
instance.
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 AbstractEventLoop.connect_read_pipe()
and AbstractEventLoop.connect_write_pipe()
methods to connect pipes.
Run subprocesses asynchronously using the subprocess
module.
coroutine AbstractEventLoop.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.argv
value of the program, assuming it is a Python script.) This is similar to the standard library subprocess.Popen
class called with shell=False and the list of strings passed as the first argument; however, where Popen
takes a single argument which is list of strings, subprocess_exec()
takes multiple string arguments.
The protocol_factory must instantiate a subclass of the asyncio.SubprocessProtocol
class.
Other parameters:
connect_write_pipe()
, or the constant subprocess.PIPE
(the default). By default a new pipe will be created and connected.connect_read_pipe()
, or the constant subprocess.PIPE
(the default). By default a new pipe will be created and connected.connect_read_pipe()
, or one of the constants subprocess.PIPE
(the default) or subprocess.STDOUT
. By default a new pipe will be created and connected. When subprocess.STDOUT
is specified, the subprocess’s standard error stream will be connected to the same pipe as the standard output stream.subprocess.Popen
without 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.Popen
class for parameters.
coroutine AbstractEventLoop.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.Popen
class called with shell=True
.
The protocol_factory must instantiate a subclass of the asyncio.SubprocessProtocol
class.
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 AbstractEventLoop.connect_read_pipe()
and AbstractEventLoop.connect_write_pipe()
methods.
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.devnull
will be used.
class asyncio.subprocess.Process
A subprocess created by the create_subprocess_exec()
or the create_subprocess_shell()
function.
The API of the Process
class was designed to be close to the API of the subprocess.Popen
class, but there are some differences:
poll()
methodcommunicate()
and wait()
methods don’t take a timeout parameter: use the wait_for()
functionwait()
method of the Process
class is asynchronous whereas the wait()
method of the Popen
class 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 returncode
attribute.
This method is a coroutine.
Note
This will deadlock when using stdout=PIPE
or stderr=PIPE
and 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 BrokenPipeError
or ConnectionResetError
exception 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 None
in the result tuple, you need to give stdout=PIPE
and/or stderr=PIPE
too.
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 BrokenPipeError
and ConnectionResetError
.
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. On Posix OSs the method sends signal.SIGTERM
to 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 SIGKILL
to the child. On Windows kill()
is an alias for terminate()
.
stdin
Standard input stream (StreamWriter
), None
if the process was created with stdin=None
.
stdout
Standard output stream (StreamReader
), None
if the process was created with stdout=None
.
stderr
Standard error stream (StreamReader
), None
if the process was created with stderr=None
.
Warning
Use the communicate()
method rather than .stdin.write
, .stdout.read
or .stderr.read
to 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 None
value indicates that the process has not terminated yet.
A negative value -N
indicates that the child was terminated by signal N
(Unix only).
asyncio supports running subprocesses from different threads, but there are limits:
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.
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 AbstractEventLoop.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()
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()
© 2001–2017 Python Software Foundation
Licensed under the PSF License.
https://docs.python.org/3.5/library/asyncio-subprocess.html