mypy-play

# remove the typing hints and mypy fails
from typing import Callable, Union

def fun1(x: int) -> Callable[[int], int]:
    return lambda y: x + y
    
x = fun1(1)

reveal_type(x)
    

mypy-play

from typing import Union, Never

def check(x: int) -> Union[Never, str]:
    try:
        assert x == 2, "Expected 2"
        
        return "Sat"
    except AssertionError as msg:
        print(msg)

stroxler

# A quick example to illustrate the two syntaxes
def f(x: str, y: int) -> bool: ...
    

f: ((str, int) -> bool)    # full parentheses syntax
f: (str, int) -> bool      # args only parentheses

    
# Now, some samples from typeshed (I abbreviated a few args lists so that it's easier to find the relevant part)

stroxler

1. We can't naively union a callable type with another type:

union_of_callables: (int) -> str | (bool) -> str  # Syntax error!

# Needs extra parens; Not intuitive or user-friendly.
union_of_callable2: ((int) -> str) | ((bool) -> str)

# Simpler and can be easily split into multiple lines, as Никита mentioned.

JDevlieghere

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stroxler

Some existing Callable types from github

Using the most restrictive full-stub syntax, here's how we'd express a few Callable types:

# from functools.pyi
f0: Callable[[_T1, _T2], S)
f0: (t1: _T1, t2: _T2, /) -> S

# from shapeflow/api.py
f1: Callable[[Optional[int], np.ndarray], stream_image]