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.