I noticed that doing set operations on an OrderedDict KeysView only works when the KeysView is the first input to the expression, and not when it's the second input. This is not the case for dicts.

Python 3.4.1 (default, May 31 2014, 11:25:02) 
[GCC 4.8.2] on linux
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>>> import collections
>>> x = collections.OrderedDict()
>>> x.keys() - set()
set()
>>> set() - x.keys()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'set' and 'KeysView'

>>> y = {}
>>> y.keys() - set()
set()
>>> set() - y.keys()
set()