Gotcha: Python mutable default arguments persist between calls

Default mutable arguments (lists, dicts) in Python functions are shared across calls, leading to unexpected accumulation.

Use None as default and create the mutable object inside the function:

# BAD - list is shared across calls:
def add_item(item, lst=[]):
lst.append(item)
return lst

add_item(1) # [1]
add_item(2) # [1, 2] — unexpected!

# GOOD:
def add_item(item, lst=None):
if lst is None:
lst = []
lst.append(item)
return lst

Default arguments are evaluated once at function definition time, not at each call. Mutable defaults become shared state.