Why were dictionary magnitude comparisons removed?

Question

Just curious, nothing more. Why were dictionary magnitude comparisons (> < >= <=) removed in Python3? What's the reason that brought to delete them?

For example: dictA > dictB

I'm guessing it's something like `if dict_a > dict_b:`, which runs in Python 2, but gives `TypeError: unorderable types` in Python 3. — Kevin, Jul 22 '14 at 18:24
You could try searching the relevant commit history, which is public. It may contain a rationale, a link to a PEP, or a reference to a mailing list discussion. — Dietrich Epp, Jul 22 '14 at 18:25
You mean the [simplification of ordering comparisons](https://docs.python.org/3/whatsnew/3.0.html#ordering-comparisons)? That's been done in Python 3.0. — Martijn Pieters, Jul 22 '14 at 18:26
If so, then this is closely related to [Is everything greater than None?](http://stackoverflow.com/q/2214194) and other such posts. — Martijn Pieters, Jul 22 '14 at 18:30
@MartijnPieters I mean why I can't `dictA > dictB` or (`< >= <=`) — zer0uno, Jul 22 '14 at 18:48

score 3 · Accepted Answer · answered Jul 22 '14 at 18:54

3

Arbitrary comparison ordering was removed from Python 3, see Ordering Comparisons in the What's New in Python 3.0 documentation.

There is no meaningful natural ordering between dictionaries. Python 2 only pretended there was to play nice with sorting mixed lists, but this only led to enormous confusion.

Take comparing strings with integers for example; integers are always smaller than strings in Python:

>>> 10 < "10"
True

Many a beginner will try to compare strings with integers anyway; it is natural to use number = raw_input('Pick a number! ') and then try to compare this with an integer; and sometimes this will look like it is working! In Python 3 this is now an error.

The same applies to the majority of objects; unless they explicitly define comparison methods (__lt__, __gt__, etc.) the types are not orderable. This includes dictionaries.

answered Jul 22 '14 at 18:54

Martijn Pieters

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According to "Learning Python" by Mark Lutz: "magnitude comparisons for dictionaries are removed in Python 3.X because they incur too much overhead when equality is desired". So I don't get, were they removed for meaningful reason or performance reason? – zer0uno Jul 22 '14 at 18:59
@antox: The Python 2 comparison code was rather arbitrary; there is no natural order for dictionaries. I don't agree with Lutz's assertion as to why this was removed. – Martijn Pieters Jul 22 '14 at 19:04
I don't know if it can help but it continues saying: "...when eqaulity is desired (equality uses an optimized scheme in 3.X that doesn't literally compare sorted key/value lists)" – zer0uno Jul 22 '14 at 19:06
@antox: The [Python 3 version of `dict_equal()`](http://hg.python.org/cpython/file/bf1b0afe419b/Objects/dictobject.c#l2130) is essentially no different from the [version in Python 2](http://hg.python.org/cpython/file/5549075d6223/Objects/dictobject.c#l1840); it is only the `dict_compare` implementation in Python 2 that is really different here (and absent in Py 3). – Martijn Pieters Jul 22 '14 at 20:45

Why were dictionary magnitude comparisons removed?

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