Calling into Trio

Executing async coroutines in Trio from Kivy

Kivy is a GUI framework that runs an event loop that calls functions synchronously upon interactions with the GUI. Some applications also run a Trio eventloop executing async code in the Kivy thread (when kivy is run asynchronously) or in a separate thread. This package enables Kivy to execute an async coroutine in the trio context.

E.g. after the trio and kivy kivy_trio.context is initialized, the following async function that sends a message to a device:

@kivy_run_in_async_quiet
async def send_device_message(delay, device, message):
    await trio.sleep(delay)
    await device.send(message)

can be scheduled to run in Trio from within a kivy context simply by calling it:

dev = MyDevice()
send_device_message(3, dev, 'hello')

This uses the kivy_run_in_async_quiet() to automatically schedule the async function to run in the trio context by wrapping it in a synchronous decorator. Then, when send_device_message is called by Kivy, Kivy doesn’t block waiting for the function to finish.

kivy_run_in_async_quiet() can only be used for async functions or methods that cannot raise an exception and when we don’t need a return value. Using it on functions that can raise an exception is unsafe as it will crash the program.

Instead, kivy_run_in_async() can be used for general functions and methods as in the following modified async function example:

async def send_device_message(delay, device, message):
    await trio.sleep(delay)
    return await device.send(message)

Now, we also need to define a synchronous wrapper function that handles the return value and potential exceptions as follows:

@kivy_run_in_async
def kivy_send_message(delay, device, message):
    try:
        response = yield mark(
            send_device_message, delay, device, message=message)
        print(f'Device responded with {response}')
    except ValueError as e:
        print(f'Device failed with error {e}')

This is similarly scheduled to run in a trio context from within a kivy context by calling it:

dev = MyDevice()
kivy_send_message(3, dev, 'hello')

kivy_run_in_async() performs the following actions when called as event = kivy_send_message(...) in three phases.

1. First kivy_run_in_async() instantiates the underlying kivy_send_message generator to get the yielded value. This advances kivy_send_message internally to the yield point. The value the wrapper function must yield to the decorator is a mark()-ed up async function and the positional/keyword arguments that will be passed to it.

2. Next, the kivy_run_in_async() decorator immediately schedules the marked async function to be called in Trio, with the wrapper suspended at the yield point.

3. Finally, when the async function has finished or raised an exception in Trio, using the kivy clock the generator is resumed from the yield point by the decorator. It then finishes executing the wrapper function by yielding the return value of the wrapped async function or re-raises its exception.

Consequently, the marked async function is executed in Trio, but the return value is then passed back or any exception the async function has raised is similarly re-raised in the kivy context when the generator resumes.

Exceptions

If the async coroutine raises an exception, the exception will be caught and sent to resume the wrapper generator at the yield statement so the wrapper will raise that exception and be given an opportunity to handle it. So e.g.:

async def raise_error():
    raise ValueError('Goodbye')

@kivy_run_in_async
def kivy_send_message():
    try:
        response = yield mark(raise_error)
    except ValueError as e:
        print(f'Error caught {e}')

when kivy_send_message is called, it’ll catch the exception raised in trio and print 'Error caught Goodbye'. If the exception is not caught, then it’ll crash Kivy because it’s the Kivy Clock that is actually executing it.

If kivy_run_in_async_quiet() was used instead to decorate raise_error directly, the Kivy event loop will crash, so it is only to be used for functions that can’t raise exceptions.

Lifecycle and Cancellation

A kivy_run_in_async() and kivy_run_in_async_quiet() decorated function or method may only be called while the Kivy event loop and trio event loop are running. Otherwise, an exception may be raised when the function is called.

If the kivy event loop ends while the coroutine is executing in trio, such as when the Kivy GUI exits, the event will be canceled and a KivyEventCancelled exception will be injected into the wrapper generator. The coroutine will still finish executing in trio and there’s currently no way to cancel that once it started, but the result will be discarded when it’s done.

A waiting event may be explicitly canceled with KivyCallbackEvent.cancel(). As above a KivyEventCancelled exception will be injected into the generator and the coroutine will still finish executing in trio, but its result will be discarded.

E.g. given the following functions:

async def send_device_message(delay, device, message):
    await trio.sleep(delay)
    result = await device.send(message)
    return result

@kivy_run_in_async
def kivy_send_message(delay, device, message):
    try:
        response = yield mark(
            send_device_message, delay, device, message=message)
        print(f'Device responded with {response}')
    except KivyEventCancelled:
        print('Event canceled')

then if we do:

>>> dev = MyDevice()
>>> event = kivy_send_message(3, dev, 'hello')
>>> # a little later in kivy
>>> event.cancel()

this will print Event canceled.

Threading

A kivy_run_in_async() and kivy_run_in_async_quiet() decorated function is only safe to be called from the kivy thread, and generally only if the kivy_trio.context was properly initialized (if kivy and trio share the same thread, initialization is not stricly nessecary, but it does increase performance considerably). The coroutine will be executed in the trio context that it was initialized to, which can be the same or another thread.

See the kivy_trio.context for details.

class kivy_trio.to_trio.KivyCallbackEvent(clock, func, gen, ret_func)

Bases: object

The event class returned when a kivy_run_in_async() or kivy_run_in_async_quiet() decorated function is called. It represents the async code scheduled to be called by trio.

This class should not be instantiated manually in user code.

E.g. the following code prints the event as it’s scheduled:

import trio
from kivy.app import App
from kivy.lang import Builder
from kivy_trio.to_trio import kivy_run_in_async_quiet, kivy_run_in_async, mark
from kivy_trio.context import initialize_shared_thread

kv = """
BoxLayout:
    Button:
        text: "Simple async"
        on_release: print(f'Event: {app.do_something_simple_async()}')
    Button:
        text: "async with error checking"
        on_release: print(f'Event: {app.call_async()}')
"""

class DemoApp(App):

    def build(self):
        return Builder.load_string(kv)

    @kivy_run_in_async_quiet
    async def do_something_simple_async(self):
        await trio.sleep(.1)
        print('Slept')

    async def do_async(self):
        await trio.sleep(1)
        print('Slept')
        raise ValueError('Woke up')

    @kivy_run_in_async
    def call_async(self):
        try:
            yield mark(self.do_async)
        except ValueError as e:
            print(f'Got error "{e}"')

    def on_start(self):
        initialize_shared_thread()

trio.run(DemoApp().async_run, 'trio')

cancel(*args)

Cancels the waiting event from reporting back to the kivy thread.

The async code itself is immediately scheduled to be executed from the async context so that can’t be canceled once started and it will finish executing unless that is manually canceled using trio’s mechanisms. However, cancel() allows canceling the generator if kivy_run_in_async() was used. Meaning, canceling will inject a KivyEventCancelled exception into the waiting generator. If the async code hasn’t started executing then it will also be canceled. However, we don’t provide visibility to the user whether the async code has started.

The injected KivyEventCancelled will be caught by cancel so that the exception will not be propagated beyond cancel() and it won’t cause an app crash.

Warning

This is only safe to be called from the kivy thread.

E.g. in the following code we schedule a async sleep event for 5s and then cancel it after 0.5s. In one button we only cancel the generator while the other uses a trio mechanism to also manually cancel the waiting trio code:

import trio
from kivy.app import App
from kivy.lang import Builder
from kivy.clock import Clock
from kivy_trio.to_trio import kivy_run_in_async, mark, KivyEventCancelled
from kivy_trio.context import initialize_shared_thread

kv = """
BoxLayout:
    Button:
        text: "async without full cancel"
        on_release: app.start_and_cancel_async(False)
    Button:
        text: "async with full cancel"
        on_release: app.start_and_cancel_async(True)
"""

class DemoApp(App):

    # scope we use to manually cancel async code from trio
    _cancel_scope = None

    def build(self):
        return Builder.load_string(kv)

    async def do_async(self, allow_cancel):
        print('starting to async sleep')
        if allow_cancel:
            # if we want to be able to cancel trio's code, we need
            # to create a scope that can be canceled
            with trio.CancelScope() as self._cancel_scope:
                try:
                    await trio.sleep(5)
                except trio.Cancelled:
                    print('Async sleeping code was canceled by trio')
            self._cancel_scope = None
        else:
            await trio.sleep(5)
            print('Finished async sleeping')

    @kivy_run_in_async
    def call_async(self, allow_cancel):
        try:
            yield mark(self.do_async, allow_cancel=allow_cancel)
        except KivyEventCancelled:
            # catch the cancellation event
            print('Generator waiting for async to finish was canceled')
            if self._cancel_scope is not None:
                # we can call trio's cancel because it's running in kivy thread
                self._cancel_scope.cancel()
        print('Generator finished')

    def start_and_cancel_async(self, allow_cancel):
        event = self.call_async(allow_cancel)
        Clock.schedule_once(event.cancel, .5)

    def on_start(self):
        initialize_shared_thread()

trio.run(DemoApp().async_run, 'trio')

Return type

None

exception kivy_trio.to_trio.KivyEventCancelled

Bases: BaseException

The exception raised in the waiting kivy_run_in_async() decorated generator if the event is canceled or if one of the event loops exit before doing the callback.

E.g.:

@kivy_run_in_async
def trigger_async_error(self):
    try:
        yield mark(some_func)
    except KivyEventCancelled:
        # event was canceled while some_func was waiting to be called
        pass

kivy_trio.to_trio.kivy_run_in_async(gen)

Decorator for a generator that yields an async coroutine. When the decorated generator is called in synchronous code from Kivy, it schedules the async coroutine in trio and returns a KivyCallbackEvent representing the scheduled code.

If the async coroutine cannot cause an exception, you don’t need its return value, and we don’t care if it’s canceled with cancel() while we still wait, you can use kivy_run_in_async_quiet() instead to decorate the async code directly.

E.g. to execute async code in trio with passed parameters and then catch the exception it raises:

import trio
from kivy.app import App
from kivy.lang import Builder
from kivy_trio.to_trio import kivy_run_in_async, mark
from kivy_trio.context import initialize_shared_thread

class DemoApp(App):

    def build(self):
        return Builder.load_string(
            "Button:\n"
            "    text: 'Press me'\n"
            "    on_release: app.call_async(5)")

    async def do_async(self, duration):
        print(f'Sleeping for {duration}s')
        await trio.sleep(duration)
        raise ValueError('Woke up')

    @kivy_run_in_async
    def call_async(self, duration):
        try:
            yield mark(self.do_async, duration=duration)
        except ValueError as e:
            print(f'Got exception "{e}"')

    def on_start(self):
        initialize_shared_thread()

trio.run(DemoApp().async_run, 'trio')

When run and pressing a button and waiting, this prints:

Sleeping for 5s
Got exception "Woke up"

Similarly, by changing do_async and call_async to the following:

async def do_async(self, duration):
    print(f'Sleeping for {duration}s')
    await trio.sleep(duration)
    return duration

@kivy_run_in_async
    def call_async(self, duration):
        result = yield mark(self.do_async, duration=duration)
        print(f'Slept for {result}')

the async code won’t raise an exception and instead returns the slept time to the generator and when run it prints:

Sleeping for 5s
Slept for 5

The generator can also catch exceptions due to the user canceling the event with cancel() (see that method for details) or if the Kivy event loop closes before the async code is done.

The following example demonstrates the cancellation that automatically happens when Kivy ends while the async code is still waiting. Run the code, press the button and then exit the window while it’s sleeping:

import trio
from kivy.app import App
from kivy.lang import Builder
from kivy_trio.to_trio import kivy_run_in_async, mark, KivyEventCancelled
from kivy_trio.context import initialize_shared_thread

class DemoApp(App):

    def build(self):
        return Builder.load_string(
            "Button:\n"
            "    text: 'Press me'\n"
            "    on_release: app.call_async(5)")

    async def do_async(self, duration):
        print(f'Sleeping for {duration}s')
        await trio.sleep(duration)

    @kivy_run_in_async
    def call_async(self, duration):
        try:
            yield mark(self.do_async, duration=duration)
        except KivyEventCancelled as e:
            print(f'Got canceled exception "{e}"')

    def on_start(self):
        initialize_shared_thread()

trio.run(DemoApp().async_run, 'trio')

When run like that it should print:

Sleeping for 5s
Got canceled exception "Event was canceled"

Return type

Callable[..., KivyCallbackEvent]

kivy_trio.to_trio.kivy_run_in_async_quiet(async_func)

Decorator for an async coroutine that schedules the async coroutine in trio and returns a KivyCallbackEvent representing the scheduled code.

It’s similar to kivy_run_in_async(), however unlike that function that can catch exceptions and the async coroutine’s return value, this will only execute the async coroutine without any protection or return value.

kivy_run_in_async_quiet() should only be used if the async coroutine cannot cause an exception (as it can’t be caught and will crash the program), you don’t need its return value, and we don’t care if it’s canceled with cancel() while we still wait (because you won’t know) and Kivy won’t be stopped while we wait (that will raise a KivyEventCancelled in Kivy as it exits).

E.g.:

import trio
from kivy.app import App
from kivy.lang import Builder
from kivy_trio.to_trio import kivy_run_in_async_quiet
from kivy_trio.context import initialize_shared_thread

class DemoApp(App):

    def build(self):
        return Builder.load_string(
            "Button:\n"
            "    text: 'Press me'\n"
            "    on_release: app.call_async(5)")

    @kivy_run_in_async_quiet
    async def call_async(self, duration):
        print(f'Sleeping for {duration}s')
        await trio.sleep(duration)

    def on_start(self):
        initialize_shared_thread()

trio.run(DemoApp().async_run, 'trio')

Return type

Callable[..., KivyCallbackEvent]

kivy_trio.to_trio.mark(__async_func, *args, **kwargs)

Collects the function and its parameters in a kivy_run_in_async() decorated generator. The function is subsequently called and awaited by trio, passing in its positional and keyword arguments.

E.g. in the code below, mark() collects the function, positional, and keyword arguments and yields it to be called and awaited by trio:

async def call_server(name, ip, port):
    if name == "Unknown":
        raise ValueError

@kivy_run_in_async
def on_button_press(self, name, ip=0.0.0.0, port=6768):
    try:
        yield mark(call_server, name, ip=ip, port=port)
    except ValueError:
        # async call_server function failed with ValueError
        pass

Parameters

• __async_func (Callable[..., Awaitable]) – The async function or method to call.

• args (Any) – Any positional arguments to be passed to the function.

• kwargs (Any) – Any keyword arguments to be passed to the function.

Return type

Tuple[Callable[..., Awaitable], Tuple[Any], Dict[str, Any]]

Returns

A collection of the functions and its args to be used internally. The exact form is internal and is not part of the public API.