On 26 Nov 2017, 12:27 PM +1100, Nathaniel Smith <njs@pobox.com>, wrote:
It turns out that the PEP 484 type system is *mostly* not useful for
this. They're really designed for checking consistency across a large
code-base, not for enabling compiler speedups. For example, if you
annotate something as an int, that means "this object is a subclass of
int". This is enough to let mypy catch your mistake if you
accidentally pass in a float instead, but it's not enough to tell you
anything at all about the object's behavior -- you could make a wacky
int subclass that acts like a string or something. 

But doesn’t Cython do all kinds of type conversions and implied equivalences that could be applied here? e.g. I’m going to annotate this as int, which might mean whatever in mypy, but if I pass this .py file to a newfangled Cython 0.35 compiler, the compiler will understand this to mean “actually really this is an int”?