Python Class Variable Mutation Trap Explained for Experienced Developers

A common Python pitfall involves mutable class variables being shared across all instances, producing unexpected behavior. When a list is defined as a class attribute, all instances reference the same object, so mutations like append affect every instance simultaneously. This differs from immutable types such as integers, where augmented assignment creates a new instance variable rather than modifying the class-level object. The correct fix is to initialize mutable attributes inside __init__, ensuring each instance gets its own copy. Class variables remain appropriate for constants, shared configuration, or carefully managed class-wide counters when modified explicitly through the class name.