[Python-Dev] Process to remove a Python feature

[Matthias Bussonnier]

On Fri, 4 May 2018 at 11:49, Serhiy Storchaka <storchaka at gmail.com> wrote:

> 04.05.18 20:57, Matthias Bussonnier пише:
> > But when I hit a DeprecationWarning message there is one crucial piece of
> > information missing most of the time: Since which version number it's
> > deprecated
> > (and sometime since when the replacement is available could be good if
> > overlap
> > between functionality there was).
>
> I think the information about since which version number it will be
> removed is more useful. Different cases need different deprecation
> periods. The more common the case, the longer deprecation period should
> be. Some recently added warnings contain this information.
>

Maybe to push people forward, but from experience it is hard to predict
future, so saying when
it _will_ be remove is hard.  When you _want_ to remove it probably.
Victor's first mail in
this thread is a good example. The functionality was marked to be removed
from 3.7, but I think
it is likely too late now. You can always update, but I hate giving
differing information between software version.

I'm curious about your use case for the version of removal,
I usually don't care when it's going to be removed, I prefer since
when the functionality is deprecated.

if pyversion < deprecated_version:
  old_stuff
else:
  new_stuff

And I soon as my project drop deprecated_version, I remove the conditional.
I do not try/except on purpose to be able to grep for when to remove the
code.

Could you share you use case ? Of be ore detailed ?

We can also be more generic and say that if DeprecationWarning messages
could contain
timeline informations it would likely encourage the migration.

One related question is how much are DeprecationWarning messages stables
between versions ?
Would any update to many of these be accepted of refused because users
might be filtering them ?
-- 
Matthias


>
> Ideally any deprecated feature should have a replacement, and this
> replacement should be available in at least one version before adding
> the deprecation warning.
>
> X.Y: added a replacement
>
> X.Y+1: added a deprecation warning. Many users need to support only two
> recent versions and can move to using the replacement now.
>
> X.Y+3 (or X.Y+2): removed the deprecated feature. Versions older than
> X.Y should grew out of use at that moment.
>
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