Support for multiple dispatch libraries at runtime with @overload

[gabrieldemarmiesse]

Hi there! Thanks for the awesome tool!

With @overload and get_overloads, Python has opened a path to implement multiple dispatch libraries that work at runtime. Similar to C++ and Julia.

I made my own by following closely the semantics of the Mypy guide on overload here: https://mypy.readthedocs.io/en/stable/more_types.html#function-overloading

Here is the library (not released on Pypi yet, but in a few days probably): https://github.com/gabrieldemarmiesse/overtake

That works with Mypy and Pycharm. For Pyright it works for the most part, I just have a small error (legitimate, I might say).

from datetime import date
from typing import overload

from overtake import overtake


@overload
def days_this_year(current_date: date) -> int:
    delta = current_date - date(2023, 1, 1)
    return delta.days


@overload
def days_this_year(current_date: str) -> int:
    return days_this_year(date.fromisoformat(current_date))


@overtake
def days_this_year(current_date):
    ...


print(days_this_year(date(2023, 8, 15)))
# 226
print(days_this_year("2023-08-15"))
# 226

$ pyright ./trying_stuff.py
/projects/open_source/overtake/trying_stuff.py
  /projects/open_source/overtake/trying_stuff.py:8:5 - error: "days_this_year" is marked as overload, but it includes an implementation
    The body of a function overload should be "..." (reportGeneralTypeIssues)
  /projects/open_source/overtake/trying_stuff.py:14:5 - error: "days_this_year" is marked as overload, but it includes an implementation
    The body of a function overload should be "..." (reportGeneralTypeIssues)
2 errors, 0 warnings, 0 informations

It can easily be fixed by a # type: ignore without impacting too much the type checking.
I am aware that this is not a false positive. I'm not asking anything of the Pyright devs now. Just to keep it in mind when implementing more checks.

If all goes well, maybe I can do a PEP so that multiple dispatch libraries like those are supported by the typing system completely. Well the support is not very far since Mypy doesn't complain at all, and Pyright doesn't complain much.

[gabrieldemarmiesse]

Update: here is the pep: https://github.com/gabrieldemarmiesse/PEP-draft-multiple-dispatcher

here is the discussion: https://discuss.python.org/t/multiple-dispatch-based-on-typing-overload/26197