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

[Nathaniel Smith]

On Nov 8, 2017 16:12, "Nick Coghlan" <ncoghlan at gmail.com> wrote:

On 9 November 2017 at 07:46, Antoine Pitrou <antoine at python.org> wrote:
>
> Le 08/11/2017 à 22:43, Nick Coghlan a écrit :
>>
>> 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
>
> Nick, have you actually read the discussion and the complaints people
> had with the current situation?  Most of them *don't* specifically talk
> about __main__ scripts.

I have, and I've also re-read the discussions regarding why the
default got changed in the first place.

Behaviour up until 2.6 & 3.1:

    once::DeprecationWarning

Behaviour since 2.7 & 3.2:

    ignore::DeprecationWarning

With test runners overriding the default filters to set it back to
"once::DeprecationWarning".


Is this intended to be a description of the current state of affairs?
Because I've never encountered a test runner that does this... Which
runners are you thinking of?


The rationale for that change was so that end users of applications
that merely happened to be written in Python wouldn't see deprecation
warnings when Linux distros (or the end user) updated to a new Python
version. It had the downside that you had to remember to opt-in to
deprecation warnings in order to see them, which is a problem if you
mostly use Python for ad hoc personal scripting.

Proposed behaviour for Python 3.7+:

    once::DeprecationWarning:__main__
    ignore::DeprecationWarning

With test runners still overriding the default filters to set them
back to "once::DeprecationWarning".

This is a partial reversion back to the pre-2.7 behaviour, focused
specifically on interactive use and ad hoc personal scripting. For ad
hoc *distributed* scripting, the changed default encourages upgrading
from single-file scripts to the zipapp model, and then minimising the
amount of code that runs directly in __main__.py.

I expect this will be a sufficient change to solve the specific
problem I'm personally concerned by, so I'm no longer inclined to
argue for anything more complicated. Other folks may have other
concerns that this tweak to the default filters doesn't address - they
can continue to build their case for more complex options using this
as the new baseline behaviour.


I think most people's concern is that we've gotten into a state where
DeprecationWarning's are largely useless in practice, because no one sees
them. Effectively the norm now is that developers (both the Python core
team and downstream libraries) think they're following some sensible
deprecation cycle, but often they're actually making changes without any
warning, just they wait a year to do it. It's not clear why we're bothering
through multiple releases -- which adds major overhead -- if in practice we
aren't going to actually warn most people. Enabling them for another 1% of
code doesn't really address this.

As I mentioned above, it's also having the paradoxical effect of making it
so that end-users are *more* likely to see deprecation warnings, since
major libraries are giving up on using DeprecationWarning. Most recently it
looks like pyca/cryptography is going to switch, partly as a result of this
thread:
  https://github.com/pyca/cryptography/pull/4014

Some more ideas to throw out there:

- if an envvar CI=true is set, then by default make deprecation warnings
into errors. (This is an informal standard that lots of CI systems use.
Error instead of "once" because most people don't look at CI output at all
unless there's an error.)

- provide some mechanism that makes it easy to have a deprecation warning
that starts out as invisible, but then becomes visible as you get closer to
the switchover point. (E.g. CPython might make the deprecation warnings
that it issues be invisible in 3.x.0 and 3.x.1 but become visible in
3.x.2+.) Maybe:

# in warnings.py
def deprecation_warning(library_version, visible_in_version,
change_in_version, msg, stacklevel):
    ...

Then a call like:

  deprecation_warning(my_library.__version__, "1.3", "1.4", "This function
is deprecated", 2)

issues an InvisibleDeprecationWarning if my_library.__version__ < 1.3, and
a VisibleDeprecationWarning otherwise.

(The stacklevel argument is mandatory because the usual default of 1 is
always wrong for deprecation warnings.)

-n
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