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--> from collections import deque
--> d = deque()
--> d += 1
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable
deque should be returning NotImplemented, which would generate a more appropriate error message:
TypeError: unsupported operand type(s) for -=: 'collections.deque' and 'int'
which is what happens with -=, *=, /=, |=, at which point I stopped testing.

Please don't replicate the same variant of the issue multiple times.

This behavior was intended. It was modeled after the behavior for lists.
Any change here would break code such as:
 d = deque()
 d += 'abc' # This is specially allowed (note, we have a test for it)