On Sat, Oct 16, 2021 at 3:18 PM MRAB <python@mrabarnett.plus.com> wrote:
On 2021-10-16 22:39, Oscar Benjamin wrote:
> I would just like to note that in SymPy we had problems a few times
> because of changes made to typing in the 3.5.x series. The problem was
> that CI tested the newest version of 3.5.x but many users were using
> older versions and many features were added in micro releases e.g.
> (random example from docs) typing.NoReturn has:
>    New in version 3.5.4.
>    New in version 3.6.2.
> What that meant is that if a contributor had used NoReturn as a type
> hint then because SymPy's CI only tested the most recent 3.5.x it
> would not be noticed in PR review that SymPy had become unimportable
> in 3.5.0.
> This was not a major problem: where I saw complaints it seemed that
> users were happy to upgrade to a newer Python 3.5.x. In general though
> those changes do not match the explanation shown above. I welcome the
> documentation around what kind of changes will be made in different
> releases but just to be clear, should this preclude things like adding
> typing.NoReturn in micro releases? (I hope so.)

At the time we justified this because the typing module was provisional (see PEP 411). The rules are somewhat relaxed for provisional modules (though I admit we probably stretched the rules a bit).
Wasn't there a change in a micro version of Python 2? I can't remember
the details, but from hindsight it was seen as a mistake that should not
be repeated.

That was in the olden days. In particular, bool() and True/False were introduced in Python 2.2.1 (https://docs.python.org/2.7/library/functions.html#bool). We learned our lesson (though not perfectly, see above :-).

--Guido van Rossum (python.org/~guido)