On: What's Coming in Python 3.8

[zephyrfalcon]

    def bar():
        x += [3]   # UnboundLocalError

This is an especially funky one. x.extend([3]) would be allowed. Presumably x += [3] is not because it expands to x = x + [3]... However, the += operator on lists works the same as extend(), i.e. it changes the list in-place.

[ben509 → zephyrfalcon]

dis.dis(bar) shows:

              0 LOAD_FAST                0 (x)
              2 LOAD_CONST               1 (3)
              4 INPLACE_ADD
              6 STORE_FAST               0 (x)
              8 LOAD_CONST               0 (None)
             10 RETURN_VALUE

So INPLACE_ADD and STORE_FAST are essentially doing x = x.__iadd__([3])