[Python-Dev] PEP 565: Show DeprecationWarning in __main__

[Nathaniel Smith]

On Nov 28, 2017 3:55 PM, "Guido van Rossum" <guido at python.org> wrote:

On Sun, Nov 19, 2017 at 5:40 AM, Nathaniel Smith <njs at 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
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