uasyncio — asynchronous I/O scheduler

This module implements a subset of the corresponding CPython module, as described below. For more information, refer to the original CPython documentation: asyncio


import uasyncio

async def blink(led, period_ms):
    while True:
        await uasyncio.sleep_ms(5)
        await uasyncio.sleep_ms(period_ms)

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

# Running on a pyboard
from pyb import LED, LED(2)))

# Running on a generic board
from machine import Pin, Pin(2)))

Core functions


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

Returns the corresponding Task object.

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

Returns the value returned by coro.


Sleep for t seconds (can be a float).

This is a coroutine.


Sleep for t milliseconds.

This is a coroutine, and a MicroPython extension.

Additional functions

uasyncio.wait_for(awaitable, timeout)

Wait for the awaitable to complete, but cancel it if it takes longer that 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.

Returns the return value of awaitable.

This is a coroutine.

uasyncio.wait_for_ms(awaitable, timeout)

Similar to wait_for but timeout is an integer in milliseconds.

This is a coroutine, and a MicroPython extension.

uasyncio.gather(*awaitables, return_exceptions=False)

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 uasyncio.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 the task by injecting a CancelledError into it. The task may or may not ignore this exception.

class Event

class uasyncio.Event

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


Returns True if the event is set, False otherwise.


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


Clear the event.


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

This is a coroutine.

class Lock

class uasyncio.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.


Returns True if the lock is locked, otherwise False.


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 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

uasyncio.open_connection(host, port)

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.

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.

uasyncio.start_server(callback, host, port, backlog=5)

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.

Returns a Server object.

This is a coroutine.

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


Close the stream.


Wait for the stream to close.

This is a coroutine.

Read up to n bytes and return them.

This is a coroutine.


Read a line and return it.

This is a coroutine.


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

This is a coroutine.

class uasyncio.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 the server.


Wait for the server to close.

This is a coroutine.

Event Loop


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

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


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


Run the event loop until stop() is called.


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


Stop the event loop.


Close the event loop.


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


The default exception handler that is called.


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