Overload EventEmitter.on type to avoid mypy errors

[whitphx]

Hi,

when you use mypy as a type checker, it emits the following error like this.

# ee.py
from pyee.base import EventEmitter

ee = EventEmitter()

@ee.on('event')
def event_handler():
    print('BANG BANG')

$ mypy --version
mypy 1.15.0 (compiled: yes)
$ mypy ee.py
ee.py:5: error: Argument 1 has incompatible type "Callable[[], Any]"; expected "Never"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

So it would be helpful if type overloads like this PR is introduced.

Thanks!

[jfhbrook]

How new is the overload feature? I'm unfamiliar and assume it's a recent addition. I'd be more than willing to tighten up the typing, if it's been available long enough.

[jfhbrook]

That said, I don't immediately see how overloading addresses that error. Can you connect the dots for me?

[jfhbrook]

In the meantime, you may consider:

# ee.py
from pyee.base import EventEmitter

ee = EventEmitter()

@ee.listens_to('event')
def event_handler():
    print('BANG BANG')

[jfhbrook]

Final comment, I unblocked the CI pipeline, and it's failing on a lint error. I'm guessing flake8 has a new rule to handle overloads - perhaps upgrading it will get that pipeline working?

[whitphx]

Thank you for the comments.

How new is the overload feature?

It's available the Python versions supported by this package, >=3.8 as https://docs.python.org/3.8/library/typing.html#typing.overload
Looks like it's been available since Python 3.5 in which the typing module itself was added.

I don't immediately see how overloading addresses that error.

I think,
unlike pyright, mypy considers that the usage of @ee.on('event') without the second argument doesn't match the current type definition below and deals with its return type as Never

    def on(
        self, event: str, f: Optional[Handler] = None
    ) -> Union[Handler, Callable[[Handler], Handler]]:

so the proposed overload definition as below may fix it because mypy treats it as a matched type def for @ee.on('event')

def on(self, event: str) -> Callable[[Handler], Handler]: ...

This mypy behavior looks weird though.

In the meantime, you may consider:

It worked as expected. Thank you!

it's failing on a lint error

Will see it.

[jfhbrook]

It's available the Python versions supported by this package, >=3.8 as https://docs.python.org/3.8/library/typing.html#typing.overload
Looks like it's been available since Python 3.5 in which the typing module itself was added.

Strange, I could have sworn I hit a wall when trying to figure out overloading when I implemented typing. But if this is true, I'm glad to pull it in.

I think, unlike pyright, mypy considers that the usage of @ee.on('event') without the second argument doesn't match the current type definition below and deals with its return type as Never

This sounds like a bug in mypy - classic. Not that I'm against fixing it, but... man.

As an aside, it looks like type checking has bigger problems. I don't know when I'll have a chance to push this PR through, though.

[jfhbrook]

#177 rolls in this change, and also fixes a bunch of other issues with type checking in mypy - as well as an issue with recent versions of pyright.

The biggest issue with #177 is that it adds the Self type to make pyright happy - this breaks Python 3.8, 3.9 and 3.10. I may drop those versions and cut a major release, but I haven't decided yet. The alternative is to define Self = Any instead of importing it.

I have some vacation time coming up next week, so I should be able to make a decision and cut a release there.

I'm going to close this with the understanding that #177 supercedes it. Thanks for flagging!

[jfhbrook]

@whitphx Released in v13.0.0. Cheers! 🥂

[whitphx]

Thank you so much!