Python dict key ordering

Due to dicts preserving the order of key insertion, they’re another example of objects that may have == True but with other methods yielding different values: >>> x = dict(a=1, b=2) >>> y = dict(b=2, a=1) >>> x == y True >>> list(x) ['a', 'b'] >>> list(y) ['b', 'a'] How does Python implement remembering key insertion order? Raymond Hettinger’s 2017 talk Modern Python Dictionaries A confluence of a dozen great ideas talks about how the ordering is a side effect of other optimizations. Slides. PEP 468 – Preserving the order of **kwargs in a function (2014) points to the message where Hettinger proposed compact dicts: [Python-Dev] More compact dictionaries with faster iteration. The ActiveState link therein is dead but archive.org has a copy: https://web.archive.org/web/20190217120809/http://code.activestate.com/recipes/578375/ From that working Python prototype, we can see the order getting preserved here: def __setitem__(self, key, value): hashvalue = hash(key) index, i =…

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