The way I understand it, multiple dispatch is programming language feature
wehreby a function or method overload is chosen based on run time types.
Python already has single dispatch (functools.singledispatch). It also has
the concept of overloads for type annotations (typing.overload).
My main motivation for suggesting multiple dispatch for Python is to
simplify writing comparison operators and to facilitate mypy's ability to
understand them.
Python has a long history of writing comparison operators like this:
class X:
def __lt__(self, other: Any) -> bool:
if isinstance(other, X):
return self.a < other.a
return NotImplemented
Thanks to modern type annotations, multiple dispatch could allow something
like this:
def __lt__(lhs: X, rhs: X) -> bool:
return lhs.a < rhs.a
Essentially, in most cases of implementing comparison operators, the user
implements the dispatching with a little help from the interpreter thanks
to "return NotImplemented".
One benefit of double dispatch would be addressing typing annotation
problems like this: https://github.com/numpy/numpy/issues/17368. A type
checker cannot know the return type of the comparison operators. Even in
general, a type checker can't know whether two types are comparable.
Multiple dispatch is one of Julia's main selling points compared with
Python. They make some convincing arguments here:
https://www.youtube.com/watch?v=kc9HwsxE1OY.
What I'm wondering is whether there's any possibility in Python's distant
future for implementing comparison operators using double dispatch?
Besides being simpler, type checkers like mypy would have an easier time
inferring types. The major drawback is backwards compatibility. Is there
a way to craft things so that old style comparison operators could co-exist
with new style ones?
Best,
Neil
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