Connect signal to async callback

[davidbrochart]

• psygnal version: 0.11.1
• Python version: 3.13.1
• Operating System: Debian 12

Description

I'd like to connect a signal to an async callback, but it doesn't seem to be supported.

What I Did

This gives a "coroutine 'on_value_changed' was never awaited" warning:

from psygnal import Signal

# define an object with class attribute Signals
class MyObj:

    # this signal will emit a single string
    value_changed = Signal(str)

    def __init__(self, value=0):
        self._value = value

    def set_value(self, value):
        if value != self._value:
            self._value = str(value)
            # emit the signal
            self.value_changed.emit(self._value)

async def on_value_changed(new_value: str):
    print(f"The new value is {new_value!r}")

# instantiate the object with Signals
obj = MyObj()

# connect one or more callbacks with `connect`
obj.value_changed.connect(on_value_changed)

# callbacks are called when value changes
obj.set_value('hello!')  # prints: 'The new value is 'hello!'

[tlambert03]

hey @davidbrochart, nice to see you here again.

you're right, we haven't explicitly supported coroutine functions yet. #346 should handle this case, but I'll note a slight modification to what you'll need to do in your example: Because psygnal itself has no concept of an event loop (it's a "bring-your-own-event-loop" library), you would need to slightly modify your example to call obj.set_value() within the context of an asyncio event loop. So, basically just change that last line with:

async def main():
    # callbacks are called when value changes
    obj.set_value("hello!")

import asyncio
asyncio.run(main())

... and then with #346, it should work. Still need to clean up some typing. But if you have a moment to look and if you have any comments or suggestions, i'd be happy to hear them