Python 3.0 Dictionary Comparisons

The second to last point in the preceding section merits illustration. In Python 2.6 and earlier, dictionaries support magnitude comparisons, as though you were comparing sorted key/value lists:

C:\misc> c:\python26\python
>>> D1 = {'a':1, 'b':2}
>>> D2 = {'a':1, 'b':3}
>>> D1 == D2
False
>>> D1 < D2
True

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In Python 3.0, magnitude comparisons for dictionaries are removed because they incur too much overhead when equality is desired (equality uses an optimized scheme in 3.0 that doesn’t literally compare sorted key/value lists). The alternative in 3.0 is to either write loops to compare values by key or compare the sorted key/value lists manually—the items dictionary methods and sorted built-in suffice:

C:\misc> c:\python30\python
>>> D1 = {'a':1, 'b':2}
>>> D2 = {'a':1, 'b':3}
>>> D1 == D2
False
>>> D1 < D2
TypeError: unorderable types: dict() < dict()

>>> list(D1.items())
[('a', 1), ('b', 2)]
>>> sorted(D1.items())
[('a', 1), ('b', 2)]

>>> sorted(D1.items()) < sorted(D2.items())
True
>>> sorted(D1.items()) > sorted(D2.items())
False

In practice, most programs requiring this behavior will develop more efficient ways to compare data in dictionaries than either this workaround or the original behavior in Python 2.6.