Since built-in types are classes, they have methods and attributes like any other class. The dir() function lists them:
print(dir(42))
# ['__abs__', '__add__', '__and__', '__bool__', ...]The methods surrounded by double underscores are called dunder methods (short for "double underscore"). These special methods define how objects behave in various contexts.
Operators Call Dunder Methods
When Python encounters an operator like +, it doesn't perform addition directly. Instead, it calls the appropriate dunder method on the operands.
The expression 2 + 3 is equivalent to calling __add__ on the integer 2:
# These two are equivalent
result1 = 2 + 3
result2 = (2).__add__(3)
print(result1) # 5
print(result2) # 5Common Dunder Methods
| Operator | Dunder Method |
|---|---|
+ | __add__ |
- | __sub__ |
* | __mul__ |
/ | __truediv__ |
== | __eq__ |
< | __lt__ |
str() | __str__ |
len() | __len__ |
This mechanism applies to all Python operators and many built-in functions. Understanding it is essential for customizing how objects interact with Python's syntax.