How do I annotate a callable with *args and **kwargs?

user_562769 0
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I have a function which returns a function. I would like to find a proper type annotation. However, the returned function has *args and *kwargs. How is that annotated within Callable[[Parameters???], ReturnType]?

Example:

from typing import Callable
import io
import pandas as pd

def get_conversion_function(file_type: str) -> Callable[[io.BytesIO, TODO], pd.DataFrame]:
    def to_csv(bytes_, *args, **kwargs):
        return pd.read_csv(bytes_, **kwargs)
    if file_type == "csv":
        return to_csv
python-typing python
2 Answers

With the introduction of typing.Protocol, MyPy (and other compliant type checkers) support the full call syntax natively via the __call__ special method.

MyPy: Callback Protocols

Protocols can be used to define flexible callback types that are hard (or even impossible) to express using the Callable[...] syntax, such as variadic, overloaded, and complex generic callbacks. (...)

Simply define a Protocol whose __call__ method has the desired signature.

from typing import Protocol

class SomeCallable(Protocol):
      def __call__(self, a: str, b: int, *args: float, **kwargs: str) -> bytes: ...

def my_callable(a: str, b: int, *args: float, **kwargs: str) -> bytes:
    return b"Hello World"

def my_func(key: str) -> SomeCallable:
    return my_callable

Note that the Protocol's __call__ must include a self parameter. This parameter is ignored when comparing other signatures against the Protocol.

user_53499160

As I know, python's typing does not allow do that straightforwardly as stated in the docs of typing.Callable:

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.

But you could use mypy extensions like this:

from typing import Callable
from mypy_extensions import Arg, VarArg, KwArg

def foo(a: str, *args: int, **kwargs: float) -> str:
    return 'Hello, {}'.format(a)
    
def bar() -> Callable[[Arg(str, 'a'), VarArg(int), KwArg(float)], str]:
    return foo
user_137826690
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