CODEWITHSUNDEEP
pythonmagic-methodspython-3.x
crzyonez777
crzyonez777

Reputation: 1807

How can I override magic methods?

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?

Upvotes: 3

Views: 3295

Answers (3)

zhangxaochen
zhangxaochen

Reputation: 34047

In [12]: class Num(int):
    ...:      def __init__(self, number):
    ...:              self.number = number
    ...:      def __add__(self, other):
    ...:              #self.number += other*100 #why do you want to do other*100?
    ...:              return Num(self.number+
    ...:                         Num(other).number) #wrap "other" with "Num"
                                                    #in case "other" is an "int"

In [13]: num = Num(10)
    ...: print num+10
20

Actually, you don't need to override __add__ if your Num is a subclass of int. Simply call super in __init__ would suffice:

In [19]: class Num(int):
    ...:     def __init__(self, *args):
    ...:          super(Num, self).__init__(args)

In [20]: Num(10)
Out[20]: 10

In [21]: Num(10)+10
Out[21]: 20

and this way no other attribute like self.number is needed.

Upvotes: 3

Lix
Lix

Reputation: 47996

You are wanting to override the method of your class and not of one specific property.

In your implementation, num.number + 10 will not trigger your class __add__ method but rather the method of the variable you are operating on - in your case, an int.

num.number.__add__

This is why you see the output of 20 - it uses the default __add__ 10 + 10 = 20

If you want to use the method of your class, you would do it like this:

num = Num(10)
num + 10

Now you are accessing your num.__add__ method.

Upvotes: 1

freakish
freakish

Reputation: 56547

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

Upvotes: 3

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