How can I override magic methods?

Question

Today, I was reading a book about python and I got to know that there are some magic methods such as __add__ and __mul__.

But, in the book, there is no explanation on how to use them.

So, I tried to figure it out by myself. But, I couldn't figure out how to override magic methods. Here is the code I tried.

>>> class Num(int):
...     def __init__(self, number):
...             self.number = number
...     def __add__(self, other):
...             self.number += other*100
...             return self.number
... 
>>> num = Num(10)
>>> num.number + 10
20

Could anyone please help me understand how these magic methods work?

Answer 1

class Num:
    def __init__(self, number):
        self.number = number
    def __add__(self, other):
        self.number += other*100

>> num = Num(10)
>> num.number
10
>> num + 10  # note that you are adding to num, not to num.number
>> num.number
1010

That's how overriding __add__ works. Version with return:

class Num:
    def __init__(self, number):
        self.number = number
    def __add__(self, other):
        return self.number + other*100

>> num = Num(10)
>> num.number
10
>> num + 10  # again adding to num
1010
>> num.number
10

So basically when Python sees

x + y
x += y
x * y
x *= y
etc

it translates it to

x.__add__(y)
x.__iadd__(y)
x.__mul__(y)
x.__imul__(y)
etc