Classes

Classes

Classes bundle data and behavior into new object types.

Define a class when data and behavior should travel together. The initializer gives each object its starting state.

Source

class Counter:
    def __init__(self, start=0):
        self.value = start

first = Counter()
second = Counter(10)
print(first.value)
print(second.value)

Output

0
10

Methods are functions attached to the class. self is the particular object receiving the method call, so separate instances keep separate state.

Source

class Counter:
    def __init__(self, start=0):
        self.value = start

    def increment(self, amount=1):
        self.value += amount
        return self.value

first = Counter()
second = Counter(10)
print(first.increment())
print(second.increment(5))

Output

1
15

A name defined directly on the class body is a class attribute, shared by every instance. Reading falls back to the class when the instance has no attribute of that name; assigning to the class itself changes the value for every instance at once.

Source

class Counter:
    step = 1

    def __init__(self, start=0):
        self.value = start

first = Counter()
second = Counter()
print(first.step)
Counter.step = 5
print(second.step)

Output

1
5

A mutable class attribute is shared mutable state — the classic footgun. Define per-instance containers in __init__ so each object owns its own copy.

Source

class Cart:
    items = []

    def add(self, item):
        self.items.append(item)

shared_a = Cart()
shared_b = Cart()
shared_a.add("apple")
print(shared_b.items)

class FixedCart:
    def __init__(self):
        self.items = []

    def add(self, item):
        self.items.append(item)

own_a = FixedCart()
own_b = FixedCart()
own_a.add("apple")
print(own_b.items)

Output

['apple']
[]

Notes

self is the instance the method is operating on.
__init__ initializes each new object.
Class attributes are shared across instances; instance attributes belong to one object.
Put mutable defaults in __init__, not on the class body.
Use classes when behavior belongs with state; use dictionaries for looser structured data.

Run the complete example

Example code

Expected output

0
10
1
15
1
5
['apple']
[]

Execution time appears here after you run the example.

See also

Run the complete example

Example code

Expected output