asyncio — asynchronous I/O scheduler

This module provides a cooperative-multitasking scheduler for async/await coroutines, along with primitives for synchronisation (locks, events) and non-blocking TCP networking via stream readers and writers. Tasks are run concurrently on a single event loop; the currently running task yields control back to the loop with await.

Example:

import asyncio

async def blink(led, period_ms):
    while True:
        led.on()
        await asyncio.sleep_ms(5)
        led.off()
        await asyncio.sleep_ms(period_ms)

async def main(led1, led2):
    asyncio.create_task(blink(led1, 700))
    asyncio.create_task(blink(led2, 400))
    await asyncio.sleep_ms(10_000)

# Running on a pyboard
from pyb import LED
asyncio.run(main(LED(1), LED(2)))

# Running on a generic board
from machine import Pin
asyncio.run(main(Pin(1), Pin(2)))

Core functions

asyncio.create_task(coro: Coroutine) → Task

Create a new task from the given coroutine and schedule it to run.

Returns the corresponding Task object.

asyncio.current_task() → Task

Return the Task object associated with the currently running task.

asyncio.run(coro: Coroutine) → Any

Create a new task from the given coroutine and run it until it completes.

Returns the value returned by coro.

asyncio.sleep(t: float) → None

Sleep for t seconds (can be a float).

This is a coroutine.

asyncio.sleep_ms(t: int) → None

Sleep for t milliseconds.

This is a coroutine, and a MicroPython extension.

Additional functions

asyncio.wait_for(awaitable: Awaitable, timeout: float) → Any

Wait for the awaitable to complete, but cancel it if it takes longer than timeout seconds. If awaitable is not a task then a task will be created from it.

If a timeout occurs, it cancels the task and raises asyncio.TimeoutError: this should be trapped by the caller. The task receives asyncio.CancelledError which may be ignored or trapped using try...except or try...finally to run cleanup code.

Returns the return value of awaitable.

This is a coroutine.

asyncio.wait_for_ms(awaitable: Awaitable, timeout: int) → Any

Similar to wait_for but timeout is an integer in milliseconds.

This is a coroutine, and a MicroPython extension.

asyncio.gather(*awaitables, return_exceptions: bool = False) → List

Run all awaitables concurrently. Any awaitables that are not tasks are promoted to tasks.

Returns a list of return values of all awaitables.

This is a coroutine.

class Task

class asyncio.Task

This object wraps a coroutine into a running task. Tasks can be waited on using await task, which will wait for the task to complete and return the return value of the task.

Tasks should not be created directly, rather use create_task to create them.

cancel() → None

Cancel the task by injecting asyncio.CancelledError into it. The task may ignore this exception. Cleanup code may be run by trapping it, or via try ... finally.

class Event

class asyncio.Event

Create a new event which can be used to synchronise tasks. Events start in the cleared state.

is_set() → bool

Returns True if the event is set, False otherwise.

set() → None

Set the event. Any tasks waiting on the event will be scheduled to run.

Note: This must be called from within a task. It is not safe to call this from an IRQ, scheduler callback, or other thread. See ThreadSafeFlag.

clear() → None

Clear the event.

wait() → None

Wait for the event to be set. If the event is already set then it returns immediately.

This is a coroutine.

class ThreadSafeFlag

class asyncio.ThreadSafeFlag

Create a new flag which can be used to synchronise a task with code running outside the asyncio loop, such as other threads, IRQs, or scheduler callbacks. Flags start in the cleared state.

set() → None

Set the flag. If there is a task waiting on the flag, it will be scheduled to run.

clear() → None

Clear the flag. This may be used to ensure that a possibly previously-set flag is clear before waiting for it.

wait() → None

Wait for the flag to be set. If the flag is already set then it returns immediately. The flag is automatically reset upon return from wait.

A flag may only be waited on by a single task at a time.

This is a coroutine.

class Lock

class asyncio.Lock

Create a new lock which can be used to coordinate tasks. Locks start in the unlocked state.

In addition to the methods below, locks can be used in an async with statement.

locked() → bool

Returns True if the lock is locked, otherwise False.

acquire() → bool

Wait for the lock to be in the unlocked state and then lock it in an atomic way. Only one task can acquire the lock at any one time.

This is a coroutine.

release() → None

Release the lock. If any tasks are waiting on the lock then the next one in the queue is scheduled to run and the lock remains locked. Otherwise, no tasks are waiting an the lock becomes unlocked.

TCP stream connections

asyncio.open_connection(host: str, port: int, ssl: ssl.SSLContext | bool | None = None) → Tuple[Stream, Stream]

Open a TCP connection to the given host and port. The host address will be resolved using socket.getaddrinfo, which is currently a blocking call. If ssl is a ssl.SSLContext object, this context is used to create the transport; if ssl is True, a default context is used.

Returns a pair of streams: a reader and a writer stream. Will raise a socket-specific OSError if the host could not be resolved or if the connection could not be made.

This is a coroutine.

asyncio.start_server(callback: Callable, host: str, port: int, backlog: int = 5, ssl: ssl.SSLContext | None = None) → Server

Start a TCP server on the given host and port. The callback will be called with incoming, accepted connections, and be passed 2 arguments: reader and writer streams for the connection.

If ssl is a ssl.SSLContext object, this context is used to create the transport.

Returns a Server object.

This is a coroutine.

class asyncio.Stream

This represents a TCP stream connection. To minimise code this class implements both a reader and a writer, and both StreamReader and StreamWriter alias to this class.

get_extra_info(v: str) → Any

Get extra information about the stream, given by v. The valid values for v are: peername.

close() → None

Close the stream.

wait_closed() → None

Wait for the stream to close.

This is a coroutine.

read(n: int = -1) → bytes

Read up to n bytes and return them. If n is not provided or -1 then read all bytes until EOF. The returned value will be an empty bytes object if EOF is encountered before any bytes are read.

This is a coroutine.

readinto(buf: bytearray | memoryview) → int

Read up to n bytes into buf with n being equal to the length of buf.

Return the number of bytes read into buf.

This is a coroutine, and a MicroPython extension.

readexactly(n: int) → bytes

Read exactly n bytes and return them as a bytes object.

Raises an EOFError exception if the stream ends before reading n bytes.

This is a coroutine.

readline() → bytes

Read a line and return it.

This is a coroutine.

write(buf: bytes) → None

Accumulated buf to the output buffer. The data is only flushed when Stream.drain is called. It is recommended to call Stream.drain immediately after calling this function.

drain() → None

Drain (write) all buffered output data out to the stream.

This is a coroutine.

class asyncio.Server

This represents the server class returned from start_server. It can be used in an async with statement to close the server upon exit.

close() → None

Close the server.

wait_closed() → None

Wait for the server to close.

This is a coroutine.

Event Loop

asyncio.get_event_loop() → Loop

Return the event loop used to schedule and run tasks. See Loop.

asyncio.new_event_loop() → Loop

Reset the event loop and return it.

Note: since MicroPython only has a single event loop this function just resets the loop’s state, it does not create a new one.

class asyncio.Loop

This represents the object which schedules and runs tasks. It cannot be created, use get_event_loop instead.

create_task(coro: Coroutine) → Task

Create a task from the given coro and return the new Task object.

run_forever() → None

Run the event loop until stop() is called.

run_until_complete(awaitable: Awaitable) → Any

Run the given awaitable until it completes. If awaitable is not a task then it will be promoted to one.

stop() → None

Stop the event loop.

close() → None

Close the event loop.

set_exception_handler(handler: Callable) → None

Set the exception handler to call when a Task raises an exception that is not caught. The handler should accept two arguments: (loop, context).

get_exception_handler() → Callable | None

Get the current exception handler. Returns the handler, or None if no custom handler is set.

default_exception_handler(context: dict) → None

The default exception handler that is called.

call_exception_handler(context: dict) → None

Call the current exception handler. The argument context is passed through and is a dictionary containing keys: 'message', 'exception', 'future'.