Python classes explained: dunder methods, property and super
How a Python class prints itself with dunder methods, hides a function behind an attribute with @property, builds objects with @classmethod, and extends a parent class with super().
By the end of this page you will know what dunder methods are and why print(obj) calls one, how @property makes a method read like an attribute, when to use @classmethod, and what super() does in a subclass. It builds on Python decorators and the classes intro.
Python classes and dunder methods
Python runs your classes through special methods whose names start and end with double underscores, called dunder methods. Print an object and Python calls __repr__. Add two objects and it calls __add__. Compare them and it calls __eq__. You never call them yourself: you define them, and the syntax calls them for you.
Here is the whole example. It is used for every section below.
class Animal:
def __init__(self, name):
self.name = name
def __repr__(self):
return f"{type(self).__name__}({self.name!r})"
@property
def shout(self):
return self.name.upper() + "!"
@classmethod
def unnamed(cls):
return cls("Nobody")
class Dog(Animal):
def __init__(self, name, tricks=0):
super().__init__(name)
self.tricks = tricks
print(Dog("Rex", 3))
print(Dog("Rex").shout)
print(Dog.unnamed())Dog('Rex')
REX!
Dog('Nobody')__init__ sets up a new object. __repr__ decides what print shows: the class name and the name, so Dog('Rex').
property: a method that reads like an attribute
Put @property above a method and callers write rex.shout with no brackets. The value is computed each time it is read, so it can never go stale. Later you can add checks inside it without changing a single caller. A property with no setter is read-only: rex.shout = "hi" raises an AttributeError.
Class methods and static methods
There are three kinds of method. A normal method gets self, the instance. A class method gets cls, the class itself, and is marked with @classmethod. A static method, marked with @staticmethod, gets neither: it is a plain function that lives inside the class.
Use a class method for alternative constructors. In the example unnamed returns cls("Nobody"), so Dog.unnamed() builds a Dog, not an Animal, because cls is Dog.
Inheritance and super
A subclass inherits everything from its parent. Dog(Animal) gets __repr__, shout and unnamed for free. When the child needs its own __init__, call super().__init__(name) to run the parent's setup first, then add the new attributes. super() means the next class up in the chain.
The Animal and Dog example, line by line
Line 1 defines Animal. Lines 2 and 3 store the name. Lines 5 and 6 define __repr__. Lines 8 to 10 make shout a property. Lines 12 to 14 make unnamed a class method. Line 16 declares Dog as a subclass of Animal. Lines 17 to 19 call super() to set the name, then add tricks. Line 21 prints a dog, which calls __repr__.
What you learned
- Dunder methods let your objects work with
print,+and==. @propertymakes a method read like an attribute.- A class method receives the class, which suits alternative constructors.
super()calls the parent so a subclass can extend instead of replace.
Common questions
Do I have to call dunder methods myself? No. You define __repr__ and Python calls it when you print the object. Calling obj.__repr__() directly works but is almost never what you want.
**What happens if I forget super().__init__() in a subclass?** The parent's setup never runs. A Dog that defines its own __init__ without it has no name, and reading it raises AttributeError: 'Dog' object has no attribute 'name'.
**When should I use @staticmethod?** When a function belongs with the class by topic but needs neither the instance nor the class. If you find yourself never using it from the class, a plain module-level function is often clearer.
Can a property be assigned to? Not unless you add a setter. Without one, p.double = 5 raises AttributeError: property 'double' of 'P' object has no setter.
How do I make two objects compare equal? Define __eq__. Without it, == on two separate instances compares identity and gives False, even when their attributes match. The @dataclass shortcut writes __eq__ for you.
Next in the series: type hints, and how to tell Python and your editor what your code expects.
This article goes with the video Python Classes Explained. Watch it on YouTube.