On Nov 28, 2017 3:55 PM, "Guido van Rossum" <guido@python.org> wrote:
On Sun, Nov 19, 2017 at 5:40 AM, Nathaniel Smith <njs@pobox.com> wrote:
Eh, numpy does use FutureWarning for changes where the same code will
transition from doing one thing to doing something else without
passing through a state where it raises an error. But that decision
was based on FutureWarning being shown to users by default, not
because it matches the nominal purpose :-). IIRC I proposed this
policy for NumPy in the first place, and I still don't even know if it
matches the original intent because the docs are so vague. "Will
change behavior in the future" describes every case where you might
consider using FutureWarning *or* DeprecationWarning, right?

We have been using DeprecationWarning for changes where code will
transition from working -> raising an error, and that *is* based on
the Official Recommendation to hide those by default whenever
possible. We've been doing this for a few years now, and I'd say our
experience so far has been... poor. I'm trying to figure out how to
say this politely. Basically it doesn't work at all. What happens in
practice is that we issue a DeprecationWarning for a year, mostly
no-one notices, then we make the change in a 1.x.0 release, everyone's
code breaks, we roll it back in 1.x.1, and then possibly repeat
several times in 1.(x+1).0 and 1.(x+2).0 until enough people have
updated their code that the screams die down. I'm pretty sure we'll be
changing our policy at some point, possibly to always use
FutureWarning for everything.

Can one of you check that the latest version of PEP 565 gets this right?

If you're asking about the the proposed new language about FutureWarnings, it seems fine to me. If you're asking about the PEP as a whole, it seems fine but I don't think it will make much difference in our case. IPython has been showing deprecation warnings in __main__ for a few years now, and it's nice enough. Getting warnings for scripts seems nice too. But we aren't rolling back changes because they broke someone's one off script – I'm sure it happens but we don't tend to hear about it. We're responding to things like major downstream dependencies that nonetheless totally missed all the warnings.

The part that might help there is evangelising popular test runners like pytest to change their defaults. To me that's the most interesting change to come out of this. But it's hard to predict in advance how effective it will be.

tl;dr: I don't think PEP 565 solves all my problems, but I don't have any objections to what it does to.

-n