Mutable default argument in __init__ trap

Question

Accepted Answer

Default argument values are evaluated once when the function is defined, not each time it is called. The [] default is created once and shared across all calls that use it. When a.items.append(1) mutates that list, b = Bag() gets the same mutated list - so b.items is [1]. The fix: use None as default and create a new list inside the body: if items is None: items = []. This is one of the most common Python gotchas.