[Python-Dev] [python-committers] Enabling depreciation warnings feature code cutoff

[Nick Coghlan]

On 9 November 2017 at 07:09, Simon Cross <hodgestar+pythondev at gmail.com> wrote:
> On Wed, Nov 8, 2017 at 10:33 PM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> For interactive use, the principle ends up being "Code you write gives
>> deprecation warnings, code you import doesn't" (which is the main
>> aspect I care about, since it's the one that semi-regularly trips me
>> up when I forget that DeprecationWarning is off by default).
>
> I with Antoine here. The idea that "code in __main__" is the set of code someone
> wrote really seems a lot like guessing (and not even very good guessing).
>
> If everyone follows the "keep __main__ small" then scripts won't automatically
> display deprecation warnings by default and so the original problem of "warnings
> are easy to miss" remains.

That's an intended outcome - the goal is to have applications that are
packaged in any way (installed Python package, importable packages
with a __main__ submodule, zip archives with a __main__.py file)
continue to silence deprecation warnings by default, as in those
cases, the folks running them are *users* of those applications,
rather than developers on them.

>
> Counter proposal -- why don't testing frameworks turn on warnings by default?
> E.g. like pytest-warnings? That way people running tests will see warnings and
> others won't unless they ask to see them.

They do. The problem is that this only works for things that actually
have test suites, which misses a lot of "Interactive Python is part of
the user experience" use cases, like CPython's own REPL, and personal
scripting on a platform like Linux or conda.

However, between them, the following two guidelines should provide
pretty good deprecation warning coverage for the world's Python code:

1. If it's in __main__, it will emit deprecation warnings at runtime
2. If it's not in __main__, it should have a test suite

Thus the future answer to "I don't want deprecation warnings at
runtime" becomes "Move it out of __main__ into an importable module,
and give it a test suite".

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia