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pythontyping

Python: What is the typing signature for print?

What is the typing signature for print or similar function which takes a variable number of arguments of any type, when defined as a Callable parameter?

Concretely, what is the declaration of output_function here?

def f(x: str, output_function=print):
    output_function(x)

Update: clarified as a Callable parameter.

Solution

  • From PEP 484

    Arbitrary argument lists can as well be type annotated, so that the definition:

    def foo(*args: str, **kwds: int): ...

    is acceptable and it means that, e.g., all of the following represent function calls with valid types of arguments:

    foo('a', 'b', 'c')
foo(x=1, y=2)
foo('', z=0)

    So print would be:

    from typing import Any, IO
def print(*args: Any, sep: str = ' ', \
          end: str = '\n', file: IO = sys.stdout, \
          flush: bool = False) -> None:

    I don't think you can apply this to a Callable though. From the docs for typing,

    There is no syntax to indicate optional or keyword arguments; such function types are rarely used as callback types. Callable[..., ReturnType] (literal ellipsis) can be used to type hint a callable taking any number of arguments and returning ReturnType