Why does += behave unexpectedly on lists?

Question

The += operator in python seems to be operating unexpectedly on lists. Can anyone tell me what is going on here?

class foo:  
     bar = []
            


        

Answer 1

The other answers would seem to pretty much have it covered, though it seems worth quoting and referring to the Augmented Assignments PEP 203:

They [the augmented assignment operators] implement the same operator as their normal binary form, except that the operation is done `in-place' when the left-hand side object supports it, and that the left-hand side is only evaluated once.

...

The idea behind augmented assignment in Python is that it isn't just an easier way to write the common practice of storing the result of a binary operation in its left-hand operand, but also a way for the left-hand operand in question to know that it should operate `on itself', rather than creating a modified copy of itself.