Josh Rosenberg added the comment:
The Python implementation of OrderedDict breaks for issue28014, at least on
3.4.3 (it doesn't raise KeyError, but if you check the repr, it's only showing
one of the two entries, because calling __getitem__ is rearranging the
OrderedDict).
>>> s = SimpleLRUCache(2)
>>> s['t1'] = 1
>>> s
SimpleLRUCache([('t1', 1)])
>>> s['t2'] = 2
>>> s
SimpleLRUCache([('t1', 1)])
>>> s
SimpleLRUCache([('t2', 2)])
Again, the OrderedDict code (in the Python case, __repr__, in the C case,
popitem) assumes __getitem__ is idempotent, and again, the violation of that
constraint makes things break. They break differently in the Python
implementation and the C implementation, but they still break, because people
are trying to force OrderedDict to do unnatural things without implementing
their own logic to ensure their violations of the dict pseudo-contract actually
works.
popitem happens to be a common cause of problems because it's logically a get
and delete combined. People aren't using it for the get feature, it's just a
convenient way to remove items from the end; if they bypassed getting and just
deleted it would work, but it's a more awkward construction, so they don't. If
they implemented their own popitem that avoided their own non-idempotent
__getitem__, that would also work.
I'd be perfectly happy with making popitem implemented in terms of pop on
subclasses when pop is overridden (if pop isn't overridden though, that's
effectively what popitem already does).
I just don't think we should be making the decision that popitem *requires*
inheritance for all dict subclasses that have (normal) idempotent __contains__
and __getitem__ because classes that violate the usual expectations of
__contains__ and __getitem__ have (non-segfaulting) problems.
Note: In the expiring case, the fix is still "wrong" if someone used popitem
for the intended purpose (to get and delete). The item popped might have
expired an hour ago, but because the fixed code bypasses __getitem__, it will
happily return the expired a long expired item (having bypassed expiration
checking). It also breaks encapsulation, returning the expiry time that is
supposed to be stripped on pop. By fixing one logic flaw on behalf of a
fundamentally broken subclass, we introduced another.
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<http://bugs.python.org/issue27275>
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