Note, the += operation is conceptually similar to update() methods with are usually duck typeable:

>>> d = {}
>>> d.update(1)
Traceback (most recent call last):
  File "<pyshell#16>", line 1, in <module>
    d.update(1)
TypeError: 'int' object is not iterable

>>> s = set()
>>> s.update(1)
Traceback (most recent call last):
  File "<pyshell#18>", line 1, in <module>
    s.update(1)
TypeError: 'int' object is not iterable

>>> dict(1)
Traceback (most recent call last):
  File "<pyshell#19>", line 1, in <module>
    dict(1)
TypeError: 'int' object is not iterable