[Python-Dev] PEP 565: Show DeprecationWarning in main (original) (raw)

Nathaniel Smith njs at pobox.com
Wed Nov 29 14:28:52 EST 2017

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.

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