Python Programming

We have seen the special constructor method __init__() earlier.

Python offers more special methods that can be used to make classes act like Python built-in types.

Here are some examples:

• __str__() is used when printing. In fact, str() invokes x.__str__(). Useful if you want for example print(Person) to return something prettier and more informative than <__main__.Person object at 0x7ffe1cf127c0>.
• __add__(), __sub__(), __mul__(), __truediv__(), __pow__() will overload their respective mathematical operators.
  • for example, x+y actually invokes x.__add__(y).
• For overloading boolean operators: __lt__(), __le__(), __gt__(), __ge__(), __eq__(), __ne__() (<, <=, >, >=, ==, !=)
• Collections:
  • __len__() (len(x) invokes x.__len__())
  • __contains__() (item in x invokes x.__contains__(item))
  • __getitem__() (x[key] invokes x.__getitem__(key))
  • __setitem__() (x[key] = item invokes x.__setitem__(key, item))
  • __iter__() is used to allow the type to be used in for loops

Example

class Vector: 
    def __init__(self, a, b): 
        self.a = a 
        self.b = b

    def __add__(self, other): 
        return Vector(self.a + other.a, self.b + other.b)

    def __str__(self): 
        return f"Vector ({self.a}, {self.b})"

v1 = Vector(1, 2) 
v2 = Vector(5, 7) 
v3 = v1 + v2
print (v3)