Python's magic methods are commonly used for operator overloading.
Python's dunder methods, also known as magic methods, are special methods that are invoked automatically when certain operators or functions are used on an object.
dunder means double under for double underscore
You can use the
dirfunction to see the magic methods inherited by a class
Case Study
Lets give an example of an Item class in python
class Item:
def __init__(self, name, price):
self.name = name
self.price = price
The __init__ method is called everytime an instance of a class is created. It is also a magic method
The __str__ method
def __str__(self):
return f'Item({self.name} for {self.price})'
The __str__ method is called whenever the class is converted into a string.
So when the print function is called on an instance of the class
>> i1 = Item("Fridge", 1000)
>> print(i1)
#... Item(Fridge for 1000)
The __repr__ method
repr performs the same function as str but is used in different situations. repr is used in debugging situations and contains a detailed description of the object.
If a
__str__method is not defined, Python will default to calling the__repr__method
The __add__ method
The __add__ method is called whenever the + operator is performed on an instance of a class. For instance
For instance
i1 = Item("Fridge", 1000)
i2 = Item("Waffles",25)
print(i1 + i2)
This code returns
Traceback (most recent call last):
File "main.py", line x, in <module>
print(i1 + i2)
TypeError: unsupported operand type(s) for +: 'Item' and 'Item'
So we declare an add function that can support operand types for +
def __add__(self, other): # other for the second operand
newItem = Item(f'({self.name},{other.name})', self.price + other.price)
return newItem
so, calling i1 + i2 above will result in
Item(Fridge,Waffles for 1025)
It is advisable for dunder_methods like this to return the same type as its operands.
For Instance
>> i1 = Item("Fridge", 1000)
>> i2 = Item("Waffles",25)
>> i3 = Item("Samsung", 500)
>> print(i1 + i2 + i3)
#... Item(Fridge,Waffles,Samsung for 1525)
There are still other arithmetic dunder_methods that are commonly used such as
| Dunder Method | Operator or Inbuilt function |
|---|---|
__add__ |
+ |
__sub__ |
- |
__mul__ |
* |
__truediv__ |
/ |
__floordiv__ |
// |
__pow__ |
** |
The __gt__ method
The eq, ne, gt, le, lt, ge methods are sets of magic methods used for comparism. Likewise
| Dunder Method | Operator or Inbuilt Function |
|---|---|
__eq__ |
== |
__ne__ |
!= |
__gt__ |
> |
__le__ |
<= |
__lt__ |
< |
__ge__ |
>= |
The drawback to overloading comparision operators is that if you overload one of them, you have to overload the whole six of them
For Example
def __gt__(self, other):
return self.price > other.price
Using the gt method to check if an item is more constly than the other.
It is best practice for your comparision overloads to return boolean values
This is just a quick overview of what magic methods are in python and how operator overloading works.
Just keep in mind that for every built-in method and operator, there is a corresponding dunder method for it
For more dunder methods check here
The complete code for my example is kept here
If you have any questions, please feel free to ask 😊 below in the comments
Top comments (1)
A more detailed look at the
__str__and__repr__methods (and why to implement them) can be found here