Dunder methods

init, repr, str, cmp, new , del, hash, nonzero, unicode, class operators

• __init__ The task of constructors is to initialize(assign values) to the data members of the class when an object of class is created.
• repr() The repr() function returns a printable representation of the given object.
• The __str__ method in Python represents the class objects as a string – it can be used for classes. The str method should be defined in a way that is easy to read and outputs all the members of the class. This method is also used as a debugging tool when the members of a class need to be checked.
• __cmp__ is no longer used.
• __mew__ Whenever a class is instantiated __new__ and __init__ methods are called. __new__ method will be called when an object is created and __init__ method will be called to initialize the object.
Copy

class A(object):
    def __new__(cls):
         print("Creating instance")
         return super(A, cls).__new__(cls)
  
    def __init__(self):
        print("Init is called")

# Creating instance
# Init is called

• __hash__()
Copy

class A(object):
    def __init__(self, a, b, c):
        self._a = a
        self._b = b
        self._c = c

    def __eq__(self, othr):
        return (isinstance(othr, type(self))
                and (self._a, self._b, self._c) ==
                    (othr._a, othr._b, othr._c))

    def __hash__(self):
        return hash((self._a, self._b, self._c))

• Rich comparison methods

__lt__, __gt__, __le__, __ge__, __eq__, and __ne__

Copy

def __lt__(self, other):
   ...
def __le__(self, other):
   ...
def __gt__(self, other):
   ...
def __ge__(self, other):
   ...
def __eq__(self, other):
   ...
def __ne__(self, other):
   ...

• __call__

object() is shorthand for object.__call__()

Copy

class Product:
    def __init__(self):
        print("Instance Created")
  
    # Defining __call__ method
    def __call__(self, a, b):
        print(a * b)
  
# Instance created
ans = Product()
  
# __call__ method will be called
ans(10, 20)

• __contains__ for i in self: