Python: How to set the default value when the parameter type is function?

Question

I am modifying a function which already have some parameters with default values.

I need to take in a function as a parameter, and it should have a default value which should be something like None, so that I can avoid using it when it is not set.

Below is a simple example, while actually None should not be used.

from collections.abc import Callable

def myfunc(x: int=0, metric_func: Callable=None):
    '''Type "None" cannot be assigned to type "function"'''
    ret = []
    if metric_func == None:
        return ret

    for i in range(10):
        ret.append(metric(x, i))
    return ret

def dot(x, y):
    return x * y

if __name__ == "__main__":
    myfunc(1, dot)

Do you really want to return an empty list when metric_func isn't provided? That seems strange. (Also look at typing.Optional.) — user2357112
– user2357112, Commented Jan 8, 2023 at 9:49
Having None is actually ok, just add proper type hint like Optional[Callable] or Callable | None — GopherM
– GopherM, Commented Jan 8, 2023 at 9:51
@user2357112 Thank you for reminding that, but it's just a simple example so that's OK. — 无风引漩
– 无风引漩, Commented Jan 8, 2023 at 16:30

ljmc · Accepted Answer · 2023-01-08 12:28:27Z

Just looking into the standard library, you're using the most common method.

For instance in heapq(1) (2) (3), bisect (1) (2), itertools (1):

def merge(*iterables, key=None, reverse=False):
    '''Merge multiple sorted inputs into a single sorted output.

    Similar to sorted(itertools.chain(*iterables)) but returns a generator,
    does not pull the data into memory all at once, and assumes that each of
    the input streams is already sorted (smallest to largest).

    >>> list(merge([1,3,5,7], [0,2,4,8], [5,10,15,20], [], [25]))
    [0, 1, 2, 3, 4, 5, 5, 7, 8, 10, 15, 20, 25]

    If *key* is not None, applies a key function to each element to determine
    its sort order.

    >>> list(merge(['dog', 'horse'], ['cat', 'fish', 'kangaroo'], key=len))
    ['dog', 'cat', 'fish', 'horse', 'kangaroo']

    '''

    # skipping some lines

    if key is None:
        ...

    # skipping the rest

If you want to type it explicitly, just create a Union with Union[Callable, None] or Callable | None if using python >= 3.10.

You should check against None with v is None not v == None.

If possible, type your Callable otherwise it defaults to Callable[..., Any].

And finally also if possible type your return value(s) where I've put <idk> markers.

def myfunc(x: int = 0, metric_func: Callable[[int, int], <idk>] | None = None) -> list[<idk>]:
    '''Type "None" cannot be assigned to type "function"'''
    ret = []
    if metric_func is None:
        return ret

    for i in range(10):
        ret.append(metric_func(x, i))
    return ret

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