I believe newer Python versions have at least deterministic iteration order (correct me if I'm wrong), while some previous ones had a non-deterministic one (non-deterministic between mutiple invocations of the interpreter). But there is also OrderedDict which iterates in assignment order.

No, you're right. I believe dict orders were made deterministic (by insertion order) in 3.6. Before that, it was undefined.

In Python 2.7 prior to 2.7.3, or Python 3 prior to 3.3, iterating a dictionary would -- if it always contained the same set of keys -- be in a consistent order across multiple runs of the same program. This was an implementation detail not to be relied on.

Starting in 2.7.3 and 3.3, the hashing algorithm applied a per-process random salt to certain built-in types, in order to mitigate a potential denial-of-service vector by crafting dictionary keys which cause hash collisions. Unless the PYTHONHASHSEED environment variable was set to a consistent value, iteration over a dictionary was no longer accidentally consistent across runs.

In 3.6, the dictionary implementation was rewritten to drastically improve performance. As a side effect, dictionary iteration became consistent again, this time determined by insertion order. In 3.6, this consistency was an implementation detail and not to be relied on.

Beginning with 3.7, dictionary iteration order is finally guaranteed by the language, independent of implementation, and goes by insertion order.

CPython interates in this fashion since 3.6, but it is not part of the Python spec.

Might be a bit contentious but it's not supposed to be relied upon

It was recently decided that as of 3.7 it will be.

(Source: https://mail.python.org/pipermail/python-dev/2017-December/1...)

Hmmmmmm.... that makes it hard for other implementations. CPython isn't the whole of the Python world. Requirements like that can be problematic for implementations like Micropython, for instance.

They asked people from the Jython, pypy and uPython community if it was ok before doing so.

The entire reason they decided to make it a standard rather than keep it as an implementation detail is to make things better for users. It’s not about just looking after cpython.

I suspect the reasoning was with CPython dominating, people would eventually make this assumption in their code, perhaps unintentionally, and reduce compatibility. Better to make it official.

The headache that having non-deterministic code causes more than makes up for that.

Among other things this makes writing tests much easier.

Python dicts have always been touted as an unordered data structure though, so yeah, if you're depending on any type of order you shouldn't be.

In 3.6 dicts are ordered (by insert order) as an implementation detail. From 3.7 on it will be a feature.

There’s a well known YouTube video which covers all this comprehensively.

Raymond Hettinger, Modern Python Dictionaries... hopefully this is the correct one. https://youtu.be/npw4s1QTmPg

Ooh, I'm no longer involved in the Python world but I love presentations by Hettinger. Thanks!