How do __iadd__ and other in-place operator methods (__isub__, __imul__) differ from their non-in-place counterparts?

__iadd__(self, other) implements the += operator and can mutate self in place and return self, avoiding creating a new object (unlike __add__, which must return a new object). If __iadd__ isn't defined, Python falls back to __add__ followed by reassignment (x = x + y), which is less efficient for mutable, resizable containers.

class Bag:
    def __init__(self, items=None):
        self.items = items or []
    def __iadd__(self, other):
        self.items.extend(other)
        return self  # mutate and return self

b = Bag([1,2])
b += [3,4]
print(b.items)  # [1,2,3,4]