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

Nathaniel Smith njs at pobox.com
Sun Nov 19 08:40:30 EST 2017

On Sun, Nov 19, 2017 at 2:26 AM, Serhiy Storchaka <storchaka at gmail.com> wrote:

It seems to me that most of issues with FutureWarning on GitHub [1] are related to NumPy and pandas which use FutureWarning for its original nominal purpose, for warning about using programming interfaces that will change the behavior in future. This doesn't have any relation to end users unless the end user is an author of the written code.

[1] https://github.com/search?q=FutureWarning&type=Issues

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.

-n

-- Nathaniel J. Smith -- https://vorpus.org

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