Code using a variable like a function/method with ()?

Question

I am looking at an existing code base that is using a bool variable like a method, for example:

class Manager(object):

    def __init__(self):
        self._on_elect_callback = None
        self._on_revoke_callback = None
        self.... = ... (..., event_listener = self._event)

    def _event(self, type):
        on_elect_callback = self._on_elect_callback
        if type == SOME_CONSTANT:
            ....
            if on_elect_callback:
                on_elect_callback()

    def do_this(self, on_elect_function):
        self._on_elect_callback = on_elect_function
        if self....:
            on_elect_function()

Questions:

I am curious how that on_elect_callback is being used like a function with () after if condition on the last line. Isn't that some boolean variable? I searched the repo and there is no definition for that. What is it doing?
Also, I would like to set a variable in __init__ that the callback function of event can use like "hey this event type IS SOME_CONSTANT, so set the variable in __init__ to "ABCD" (or True), how can I achieve it? Is the way in the code above the way to do it?

self._on_elect_callback is set to None initially, which is considered falsy, but presumably there muse be another method somewhere which assigns some sort of callable, which is considered truthy and will pass the if. — quamrana
– quamrana, Commented May 27, 2020 at 17:30
@quamrana yes! there is a function like def do_this(self, on_elect_fn): self_on_elect_cb on_elect_function. And even in this def it does the same thing, on_elect_function(). So at this point I am not sure if these on_elect_callback and on_elect_function are being used as bool variables or some other type, and if they're variables or functions... I'm so confused. — user2465084
– user2465084, Commented May 27, 2020 at 17:34
Please include this method in the question above. Also please see my comment and the link that @JustinEzequiel posted. — quamrana
– quamrana, Commented May 27, 2020 at 17:36

kuco 23 · Accepted Answer · 2020-05-27 19:47:13Z

The self._on_elect_callback was probably going to be assigned a function as in self._on_elect_callback = lambda: "I'm a function". I think the reason you think it was meant to be a variable is because it is used in an if expression. The reason is that if self._on_elect_callback gets assigned a function the expression will evaluate to True, as functions support a method bool, which returns True by default. About Q2, sure you can, functions are objects in python, so you can pass them around to higher order functions, or assign them as variables as in the example

class Manager(object):

    def __init__(self, function):
        self._on_elect_callback = function # here we assigned the given function
        if not function: 
            # here we define a default attribute using an anonymous function
            self._on_elect_callback = lambda: "Default Function"

    def _event(self, type):
        if self._on_elect_callback:
            self._on_elect_callback()

Can you update your code snippet with proper formatting. Plus can you fix the _event() method so that it uses self._on_elect_callback

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