Python typing: how to define a function or method?

I want to annotate my function using type hints. I am running into a problem where I am using a function as input parameters (or more specifically, a class-method), and I do not know how I should define the type hint. The code is as follows:

async def get_mails(api: SomeApi,
                    all_contacts: ContactDict,
                    query_function: ???, 
                    aggregate_function: ???) -> list:
    
    customer_emails = await query_function()
        
    for customer_email in customer_emails:
        for email in await get_emails_by_customer(api, customer_email, get_employees()):
            all_contacts[customer_email].add_mail(email)
    
    return aggregate_function()

I added ??? Where I want to add the type hint for the functions.

>Solution :

I think what you are looking for is the Callable type. More info from the python docs here

Importantly, from the docs:

The subscription syntax must always be used with exactly two values: the argument list and the return type. The argument list must be a list of types or an ellipsis; the return type must be a single type.

So, for example, your function definition for get_mails might look like something like:

from typing import Callable

async def get_mails(api: SomeApi,
                    all_contacts: ContactDict,
                    query_function: Callable[[list, of, input, types], output_type], 
                    aggregate_function: Callable[[list, of, input, types], output_type]) -> list:
 ...

Alternatively, you can use a plain Callable, which is equivalent to Callable[..., Any].