Class variables

Learn to share class-level variables.

Class variable

A class variable is a variable defined directly in the class body. All instances share it. It is accessed as Class.name or instance.name.

Example: class Counter:\n count = 0. Counter.count gives the same value for all instances. An instance variable (self.x) is per instance; a class variable is shared by all. This distinction matters for sharing common state.

Class vs instance variable

A class variable is shared across all instances; an instance variable is per instance. Inside __init__, self.x is an instance variable; in the body, a direct x is a class variable.

Example: class Counter:\n count = 0\n def __init__(self):\n Counter.count += 1. Each new instance increments count. After c1 = Counter(); c2 = Counter(), Counter.count is 2. This is useful for an instance counter.

Exercise Add a class variable to Counter

Define the Counter class. Add the class variable count = 0 in the body. Add the method def __init__(self): and in the body write Counter.count += 1. Then create two instances with c1 = Counter() and c2 = Counter() and write print(Counter.count).

Show example solution

# Define the Counter class
class Counter:
    count = 0
    def __init__(self):
        Counter.count += 1

c1 = Counter()
c2 = Counter()
print(Counter.count)

Test yourself

How is a class variable accessed?
- Only via an instance
- Only via the class
- Only inside a method
- Both via instance and class
Show answer

Both via instance and class — A class variable is accessed both via the class and an instance.

Complete the class variable

class Counter:
    ____ = 0

Show answer

class Counter:
    count = 0

Match the variable types

count = 0

Class variable
self.x = 1

Instance variable
Counter.count += 1

Update class state

Next lesson: Constants

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