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The following code causes the interpreter to crash:
import _collections
class BadDeque(_collections.deque):
 def __new__(cls, *args):
 if len(args):
 return 42
 return _collections.deque.__new__(cls)
BadDeque() * 42
(The interpreter would crash also if we replaced 'BadDeque() * 42' with
'BadDeque() + _collections.deque([42])'.)
This is because deque_copy() (in Modules/_collectionsmodule.c) returns whatever
BadDeque() returned, without verifying it is a deque.
deque_repeat() assumes that deque_copy() returned a deque, and passes it to
deque_inplace_repeat(), which assumes it is a deque, and crashes.
(Similarly, deque_concat() assumes that deque_copy() returned a deque, which
is the reason for the other crash.)
ISTM it is a very unlikely corner case, so that adding a test (as well as
a NEWS.d item) for it is unnecessary.
What do you think?