Overview of Dunder Methods in Python

• Dunder methods allow Python class instances to interact with built-in functions and operators.
• The term “dunder” comes from “double underscore”, as in __str__, __add__, etc.
• These methods are often called implicitly by the language, not manually by the programmer.
• Also known as "magic methods", though they behave like regular methods under the hood.

Basic Example of Operator Overloading

1 + 2.3           # 3.3
[1, 2] + [3, 4]   # [1, 2, 3, 4]

• Python translates a + b into a.__add__(b) behind the scenes.
• You can define your own behavior for operators like +, in, ==, etc., via dunder methods.

Commonly Used Dunder Method Example: __init__

class Square:
    def __init__(self, side_length):
        print("Inside init!")
        self.side_length = side_length

sq = Square(1)
# Output: Inside init!

• __init__ sets up attributes when an object is instantiated.
• Called implicitly by Python, not manually.

Operator Overloading Using Dunder Methods

• Operators like +, in, ==, and others are backed by specific dunder methods.

  my_list = [2, 4, 6]
  3 in my_list              # False
  my_list.__contains__(3)   # False

• You can override these behaviors in custom classes by defining the corresponding dunder methods.

Example: Using __missing__ in Dict Subclasses

class DictSubclass(dict):
    def __missing__(self, key):
        print(f"Missing {key = }")

my_dict = DictSubclass()
my_dict[0] = True

if my_dict[0]:
    print("Key 0 was `True`.")
my_dict[1]
# Output:
# Key 0 was `True`.
# Missing key = 1

• __missing__ is triggered when a missing key is accessed in a dict subclass.

Practical Use: Reimplementing defaultdict

class my_defaultdict(dict):
    def __init__(self, default_factory, **kwargs):
        super().__init__(**kwargs)
        self.default_factory = default_factory

    def __missing__(self, key):
        self[key] = self.default_factory()
        return self[key]

olympic_medals = my_defaultdict(lambda: 0)
olympic_medals["Phelps"] = 28

print(olympic_medals["Phelps"])  # 28
print(olympic_medals["me"])      # 0

• This mimics collections.defaultdict, returning and storing a default value when keys are missing.

Dunder Methods Reference Tables

Object Lifecycle, Representation & Attribute Access

Comparison & Boolean Context

Item Access & Container Emulation

Arithmetic Operators

Bitwise & Numeric Conversions

Context Managers, Async & Callables