Order-preserving dicts were added as a side effect of an optimization in CPython 3.6 [0]. They were considered an implementation detail of CPython in 3.6, it wasn't until Python 3.7 that is was declared an official part of the language [1].

[0] https://docs.python.org/3/whatsnew/3.6.html#whatsnew36-compa... [1] https://mail.python.org/pipermail/python-dev/2017-December/1...