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Update cheat_sheet_py3.rst 2 properly xplain Callable #15613

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Update cheat_sheet_py3.rst 2 properly xplain Callable #15613

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3 changes: 3 additions & 0 deletions docs/source/cheat_sheet_py3.rst
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Expand Up @@ -109,8 +109,11 @@ Functions
x.anything() + 1 + "string" # no errors

# This is how you annotate a callable (function) value
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Explanation on which values are the inputs of a function and which one is the return type of the function is a little ambiguous for newbies who are skimming through the cheatsheet.

Callable[[A1, A2, A3], Rt] represents a function with three arguments with types A1, A2, and A3, respectively. The return type of the function is Rt.
x: Callable[[int, float], float] = f
def register(callback: Callable[[str], int]) -> None: ...
Callable[..., ReturnType] (literal ellipsis) can be used to type hint a callable taking any number of arguments and returning ReturnType
ReturnType can only be of a single type.

# A generator function that yields ints is secretly just a function that
# returns an iterator of ints, so that's how we annotate it
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