Why do we have callable objects in python?

Question

What is the purpose of a callable object? What problems do they solve?

Answer 1

There are areas, especially in the 'functions calling functions of function functions' where objects allow less nesting.

Consider making a classic decorator that checks an authorization level before calling a function. Using it is clear:

@check_authorization(level="Manager")
def update_price(Item, new_price):...

You could do this as nested functions:

def check_authorization(level):
     def take_params(function):
         def concrete(*args, **kwargs):
             if user_level_greater_than(level):
                 return function(*args, 
                     **kwargs)
             return None
         return concrete
     return take_params

Or you could to this as a class, which might be clearer:

  class check_authorization(object):
      def __init__(level):
         self.level = level
      def __call__(function):
          self.function = function
          return self.dec
      def dec(self, *args, **kwargs):
          if user_level_greater_than(self.level):
             return self.function(*args,v**kwargs)
          return None

Many would find this flat method more clear. Of course, I believe in cheating, because I like the signatures and metadata correct:

from dectools.dectools import make_call_if

@make_call_if
def check_authorization(function, arg, kwargs, level):
    return user_level_greater_than(level)

The callable object is a tool which is good for some known applications and may also be good for the bizarre problem real life throws at you.