Dunder Methods in Python

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 dir function 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

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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

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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

Comments

[Christian Ledermann]

A more detailed look at the __str__ and __repr__ methods (and why to implement them) can be found here