OOP

• __new__
• __init__ - called after the instance has been created by new
• __del__ - finalizer: called when the instance is about to be destroyed.
  • It is not guaranteed that del() methods are called for objects that still exist when the interpreter exits.
  • Note: del x doesn’t directly call x.del() — the former decrements the reference count for x by one, and the latter is only called when x’s reference count reaches zero.
• __repr__ - Called by the repr() built-in function to compute the “official” string representation of an object. This is typically used for debugging, so it is important that the representation is information-rich and unambiguous
• __str__ - Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object.
• bytes
• format
• lt / le / eq / ne / gt / ge
• hash
• bool

class A(object):
    def __init__(self):
        print("world")
 
class B(A):
    def __init__(self):
        print("hello")
        super().__init__()

class MySingleton:
    instance = None
 
    def __new__(cls, *args, **kwargs):
        if not isinstance(cls.instance, cls):
            cls.instance = object.__new__(cls)
        return cls.instance

class Singleton(type):
    _instances = {}
    def __call__(cls, *args, **kwargs):
        if cls not in cls._instances:
            cls._instances[cls] = super (Singleton,cls).__call__(*args, **kwargs)
        return cls._instances[cls]
 
class SerialNumber(metaclass=Singleton):

• attribute - direct access to data member of object
• property - properties are methods accessed like attributes. It gives full control on its getter, setter and deleter access.