Preserving order is achieved by this being an append only array where new items are always inserted at the end (insertion order). The first array, dk_entries, holds the entries ( of type PyDictKeyEntry) for the dictionary in the order that they were inserted. How does the Python 3.6 dictionary implementation perform better than the older one while preserving element order? Other implementations of Python must also offer an insertion ordered dictionary if they wish to be a conforming implementation of Python 3.7. This simply means that you can depend on it. 'Dict keeps insertion order' is the ruling. This is considered an implementation detail in Python 3.6 you need to use OrderedDict if you want insertion ordering that's guaranteed across other implementations of Python (and other ordered behavior ).Īs of Python 3.7, this is a guaranteed language feature, not merely an implementation detail. Update December 2017: dicts retaining insertion order is guaranteed for Python 3.7Īs of Python 3.6, for the CPython implementation of Python, dictionaries remember the order of items inserted. How does the new dictionary implementation perform better than the older one while preserving element order? Idea originally suggested by Raymond Hettinger.) (Contributed by INADA Naoki in issue 27350. The order-preserving aspect of this new implementation is considered an implementation detail and should not be relied upon (this may change in the future, but it is desired to have this new dict implementation in the language for a few releases before changing the language spec to mandate order-preserving semantics for all current and future Python implementations this also helps preserve backwards-compatibility with older versions of the language where random iteration order is still in effect, e.g. PEP 468 (Preserving the order of **kwargs in a function.) is implemented by this. The memory usage of the new dict() is between 20% and 25% smaller compared to Python 3.5. The documentation states:ĭict() now uses a “compact” representation pioneered by PyPy. It is described as a CPython implementation detail rather than a language feature. Dictionaries are insertion ordered as of Python 3.6.