[TwentyPosts]

This is semantically not the same as a sum type (as understood in the sense of Rust, which is afaik the academically accepted way)!

Python's `A | B` is a union operation, but in Rust a sum type is always a disjoint union. In Python, if `A = B = None`, then `A | B` has one possible instance.

In Rust, this sum type has two possible instances. This might not sound like a big deal, but the semantics are quite different.

[pid-1]

Sorry, I could not grok the difference, even after reading a few Rust examples.

def foo(int | None = None) ...

... just means the variable's default value is None in a function definition. But it could be either in an actual function call.

[SoothingSorbet]

There's no difference there because the types are already disjoint.

Say you wanted to define some function taking `YYMMDD | MMDDYY`. If both YYMMDD and MMDDYY are just aliases to `str`, then you gain no information, you cannot discriminate on which one it is, since the union `str | str` just reduces to `str`.

Sum types are disjointed unions, you can't just say `str | str`, the terms are wrapped in unique nominal data constructors, like:

enum Date { MMDDYY(String), YYMMDD(String) }

Then when accepting a `Date` you can discriminate which format it's in. You could do the same in Python by defining two unique types and using `MMDDYY | YYMMDD`.