How can I specify the function type in my type hints?

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As @jonrsharpe noted in a comment, this can be done with collections.abc.Callable:

from collections.abc import Callable

def my_function(func: Callable):

Note: Callable on its own is equivalent to Callable[..., Any]. Such a Callable takes any number and type of arguments (...) and returns a value of any type (Any). If this is too unconstrained, one may also specify the types of the input argument list and return type.

For example, given:

def sum(a: int, b: int) -> int: return a+b

The corresponding annotation is:

Callable[[int, int], int]

That is, the parameters are sub-scripted in the outer subscription with the return type as the second element in the outer subscription. In general:

Callable[[ParamType1, ParamType2, ..., ParamTypeN], ReturnType]

Answer from Dimitris Fasarakis Hilliard on Stack Overflow

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stackoverflow.com › questions › 37835179 › how-can-i-specify-the-function-type-in-my-type-hints

python - How can I specify the function type in my type hints? - Stack Overflow

Top answer

1 of 3

983

As @jonrsharpe noted in a comment, this can be done with collections.abc.Callable:

from collections.abc import Callable

def my_function(func: Callable):

For example, given:

def sum(a: int, b: int) -> int: return a+b

The corresponding annotation is:

Callable[[int, int], int]

That is, the parameters are sub-scripted in the outer subscription with the return type as the second element in the outer subscription. In general:

Callable[[ParamType1, ParamType2, ..., ParamTypeN], ReturnType]

2 of 3

My specific use case for wanting this functionality was to enable rich code completion in PyCharm. Using Callable didn't cause PyCharm to suggest that the object had a .__code__ attribute, which is what I wanted, in this case.

I stumbled across the types module and...

from types import FunctionType

allowed me to annotate an object with FunctionType and, voilà, PyCharm now suggests my object has a .__code__ attribute.

The OP wasn't clear on why this type hint was useful to them. Callable certainly works for anything that implements .__call__() but for further interface clarification, I submit the types module.

Bummer that Python needed two very similar modules.

Python documentation

docs.python.org › 3 › library › typing.html

typing — Support for type hints

The argument list must be a list of types, a ParamSpec, Concatenate, or an ellipsis (...). The return type must be a single type. If a literal ellipsis ... is given as the argument list, it indicates that a callable with any arbitrary parameter list would be acceptable: def concat(x: str, y: str) -> str: return x + y x: Callable[..., str] x = str # OK x = concat # Also OK · Callable cannot express complex signatures such as functions that take a variadic number of arguments, overloaded functions, or functions that have keyword-only parameters.

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