MiddleDebuggingCommonNot answered yet
Why do both instances share one items list?
Each cart should have its own items, but adding to one shows up in the other.
class Cart:
items = []
def add(self, item):
self.items.append(item)
a = Cart(); b = Cart()
a.add("apple")
print(b.items) # ['apple'] -- leaked across instances!Find and fix the bug.
items = [] is a class attribute shared by every instance, so a and b mutate the same list and b.items shows ['apple']. Fix: initialize it per-instance in __init__: def __init__(self): self.items = [].
- ✗Putting mutable state in a class attribute instead of
__init__ - ✗Thinking
self.items.appendcreates a per-instance attribute - ✗Believing
list()vs[]changes the sharing
- →Why does reading
self.itemsfind the class attribute when no instance attribute exists? - →When is a class attribute the right place for shared state?
