The seven flavors of callable objects[i.e. () operator]:

• User-defined functions: created with def statements or lambda expressions
• Built-in functions: like len or time.strftime(implemented in C)
• Built-in methods: methods implemented in C, like dict.get
• Methods: functions defined in the body of a class.
• Classes: when invoked, a class runs its __new__ method to create an instance, then __init__ to initialize it, and finally the instance is returned to the caller. Because there is no new operator in Python, calling a class is like calling a function
• Class instances: if a class defines a __call__ method, then its instances may be invoked as functions
• Generator functions: functions or methods that use the yield keyword. When called, generator functions return a generator object.