Pure Python mode does not like -> None annotations

I am trying to change my already heavily typed Python code to run in Cython. For that, it felt useful to put @cython.ccall everywhere and see what happens.

I found out that Cython does not understand the -> None function return type annotation as intended. Let’s take this dumb function.

def do_something(l):
    if len(l) == 0:
        return
    l.append(1)

This works fine with the decorator.

import cython

@cython.ccall
def do_something(l):
    if len(l) == 0:
        return
    l.append(1)

It’s type annotation is

def do_something(l: List[int]) -> None:

or

def do_something(l: List[int]) -> type(None):

but with both these type annotations, Cython complains

Error compiling Cython file:
------------------------------------------------------------
...
from typing import List, NoneType
import cython

@cython.ccall
def do_something(l: List[int]) -> type(None):
                                     ^
------------------------------------------------------------

test.pyx:5:38: Not a type

Error compiling Cython file:
------------------------------------------------------------
...
import cython

@cython.ccall
def do_something(l: List[int]) -> type(None):
    if len(l) == 0:
        return
       ^
------------------------------------------------------------

test.pyx:7:8: Return value required

Cython should know to treat None return types special.

[da-woods]

@Anaphory If you just want to make your code work, you can add @cython.returns(cython.void) as a decorator. It’d definitely be nice to infer this from None in future though.

[TeamSpen210]

NoReturn is something entirely different, it’s for a function which never returns, only raises an exception or infinitely loops.