Gotcha: Python mutable default arguments in class methods

Class instances share mutable default arguments, causing unexpected data sharing between instances.

Never use mutable defaults in function/method signatures:

# BUG: All instances share the same list!
class EventEmitter:
    def __init__(self, listeners=[]):  # Shared!
        self.listeners = listeners

a = EventEmitter()
b = EventEmitter()
a.listeners.append('click')
print(b.listeners)  # ['click'] - BUG!

# FIX: Use None and create in body
class EventEmitter:
    def __init__(self, listeners=None):
        self.listeners = listeners if listeners is not None else []

# Or with dataclass
from dataclasses import dataclass, field

@dataclass
class EventEmitter:
    listeners: list = field(default_factory=list)  # New list per instance

# Same issue with dicts, sets, and any mutable type:
# BAD
def add_item(item, cache={}):
    cache[item.id] = item  # Persists between calls!
    return cache

# GOOD
def add_item(item, cache=None):
    if cache is None:
        cache = {}
    cache[item.id] = item
    return cache

Default arguments are evaluated ONCE at function definition time, not at each call. The same mutable object is reused across all calls/instances.

Python classes and functions with collection defaults