python-peps/pep-0565.rst

PEP: 565
Title: Show DeprecationWarning in __main__
Author: Nick Coghlan <ncoghlan@gmail.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 12-Nov-2017
Python-Version: 3.7
Post-History: 12-Nov-2017, 25-Nov-2017


Abstract
========

In Python 2.7 and Python 3.2, the default warning filters were updated to hide
DeprecationWarning by default, such that deprecation warnings in development
tools that were themselves written in Python (e.g. linters, static analysers,
test runners, code generators), as well as any other applications that merely
happened to be written in Python, wouldn't be visible to their users unless
those users explicitly opted in to seeing them.

However, this change has had the unfortunate side effect of making
DeprecationWarning markedly less effective at its primary intended purpose:
providing advance notice of breaking changes in APIs (whether in CPython, the
standard library, or in third party libraries) to users of those APIs.

To improve this situation, this PEP proposes a single adjustment to the
default warnings filter: displaying deprecation warnings attributed to the main
module by default.

This change will mean that code entered at the interactive prompt and code in
single file scripts will revert to reporting these warnings by default, while
they will continue to be silenced by default for packaged code distributed as
part of an importable module.

The PEP also proposes a number of small adjustments to the reference
interpreter and standard library documentation to help make the warnings
subsystem more approachable for new Python developers.


Specification
=============

The current set of default warnings filters consists of::

    ignore::DeprecationWarning
    ignore::PendingDeprecationWarning
    ignore::ImportWarning
    ignore::BytesWarning
    ignore::ResourceWarning

The default ``unittest`` test runner then uses ``warnings.catch_warnings()``
``warnings.simplefilter('default')`` to override the default filters while
running test cases.

The change proposed in this PEP is to update the default warning filter list
to be::

    default::DeprecationWarning:__main__
    ignore::DeprecationWarning
    ignore::PendingDeprecationWarning
    ignore::ImportWarning
    ignore::BytesWarning
    ignore::ResourceWarning

This means that in cases where the nominal location of the warning (as
determined by the ``stacklevel`` parameter to ``warnings.warn``) is in the
``__main__`` module, the first occurrence of each DeprecationWarning will once
again be reported.

This change will lead to DeprecationWarning being displayed by default for:

* code executed directly at the interactive prompt
* code executed directly as part of a single-file script

While continuing to be hidden by default for:

* code imported from another module in a ``zipapp`` archive's ``__main__.py``
  file
* code imported from another module in an executable package's ``__main__``
  submodule
* code imported from an executable script wrapper generated at installation time
  based on a ``console_scripts`` or ``gui_scripts`` entry point definition

As a result, API deprecation warnings encountered by development tools written
in Python should continue to be hidden by default for users of those tools

While not its originally intended purpose, the standard library documentation
will also be updated to explicitly recommend the use of ``FutureWarning`` (rather
than ``DeprecationWarning``) for backwards compatibility warnings that are
intended to be seen by *users* of an application.

This will give the following three distinct categories of backwards
compatibility warning, with three different intended audiences:

* ``PendingDeprecationWarning``: reported by default only in test runners that
  override the default set of warning filters. The intended audience is Python
  developers that take an active interest in ensuring the future compatibility
  of their software (e.g. professional Python application developers with
  specific support obligations).
* ``DeprecationWarning``: reported by default for code that runs directly in
  the ``__main__`` module (as such code is considered relatively unlikely to
  have a dedicated test suite), but relies on test suite based reporting for
  code in other modules. The intended audience is Python developers that are at
  risk of upgrades to their dependencies (including upgrades to Python itself)
  breaking their software (e.g. developers using Python to script environments
  where someone else is in control of the timing of dependency upgrades).
* ``FutureWarning``: always reported by default. The intended audience is users
  of applications written in Python, rather than other Python developers
  (e.g. warning about use of a deprecated setting in a configuration file
  format).

Given its presence in the standard library since Python 2.3, ``FutureWarning``
would then also have a secondary use case for libraries and frameworks that
support multiple Python versions: as a more reliably visible alternative to
``DeprecationWarning`` in Python 2.7 and versions of Python 3.x prior to 3.7.


Documentation Updates
=====================

The current reference documentation for the warnings system is relatively short
on specific *examples* of possible settings for the ``-W`` command line option
or the ``PYTHONWARNINGS`` environment variably that achieve particular end
results.

The following improvements are proposed as part of the implementation of this
PEP:

* Explicitly list the following entries under the description of the
  ``PYTHONWARNINGS`` environment variable::

        PYTHONWARNINGS=error # Convert to exceptions
        PYTHONWARNINGS=always # Warn every time
        PYTHONWARNINGS=default # Warn once per call location
        PYTHONWARNINGS=module # Warn once per calling module
        PYTHONWARNINGS=once # Warn once per Python process
        PYTHONWARNINGS=ignore # Never warn

* Explicitly list the corresponding short options
  (``-We``, ``-Wa``, ``-Wd``, ``-Wm``,``-Wo``, ``-Wi``) for each of the
  warning actions listed under the ``-W`` command line switch documentation

* Explicitly list the default filter set in the ``warnings`` module
  documentation, using the ``action::category`` and ``action::category:module``
  notation

* Explicitly list the following snippet in the ``warnings.simplefilter``
  documentation as a recommended approach to turning off all warnings by
  default in a Python application while still allowing them to be turned
  back on via ``PYTHONWARNINGS`` or the ``-W`` command line switch::

        if not sys.warnoptions:
            warnings.simplefilter("ignore")

None of these are *new* (they already work in all still supported Python
versions), but they're not especially obvious given the current structure
of the related documentation.


Reference Implementation
========================

A reference implementation is available in the PR [4_] linked from the
related tracker issue for this PEP [5_].

As a side-effect of implementing this PEP, the internal warnings filter list
will start allowing the use of plain strings as part of filter definitions (in
addition to the existing use of compiled regular expressions). When present,
the plain strings will be compared for exact matches only. This approach allows
the new default filter to be added during interpreter startup without requiring
early access to the ``re`` module.


Motivation
==========

As discussed in [1_] and mentioned in [2_], Python 2.7 and Python 3.2 changed
the default handling of ``DeprecationWarning`` such that:

* the warning was hidden by default during normal code execution
* the ``unittest`` test runner was updated to re-enable it when running tests

The intent was to avoid cases of tooling output like the following::

    $ devtool mycode/
    /usr/lib/python3.6/site-packages/devtool/cli.py:1: DeprecationWarning: 'async' and 'await' will become reserved keywords in Python 3.7
      async = True
    ... actual tool output ...

Even when `devtool` is a tool specifically for Python programmers, this is not
a particularly useful warning, as it will be shown on every invocation, even
though the main helpful step an end user can take is to report a bug to the
developers of ``devtool``.

The warning is even less helpful for general purpose developer tools that are
used across more languages than just Python, and almost entirely \*un\*helpful
for applications that simply happen to be written in Python, and aren't
necessarily intended for a developer audience at all.

However, this change proved to have unintended consequences for the following
audiences:

* anyone using a test runner other than the default one built into ``unittest``
  (the request for third party test runners to change their default warnings
  filters was never made explicitly, so many of them still rely on the
  interpreter defaults that are designed to suit deployed applications)
* anyone using the default ``unittest`` test runner to test their Python code
  in a subprocess (since even ``unittest`` only adjusts the warnings settings
  in the current process)
* anyone writing Python code at the interactive prompt or as part of a directly
  executed script that didn't have a Python level test suite at all

In these cases, ``DeprecationWarning`` ended up become almost entirely
equivalent to ``PendingDeprecationWarning``: it was simply never seen at all.


Limitations on PEP Scope
========================

This PEP exists specifically to explain both the proposed addition to the
default warnings filter for 3.7, *and* to more clearly articulate the rationale
for the original change to the handling of DeprecationWarning back in Python 2.7
and 3.2.

This PEP does not solve all known problems with the current approach to handling
deprecation warnings. Most notably:

* the default ``unittest`` test runner does not currently report deprecation
  warnings emitted at module import time, as the warnings filter override is only
  put in place during test execution, not during test discovery and loading.
* the default ``unittest`` test runner does not currently report deprecation
  warnings in subprocesses, as the warnings filter override is applied directly
  to the loaded ``warnings`` module, not to the ``PYTHONWARNINGS`` environment
  variable.
* the standard library doesn't provide a straightforward way to opt-in to seeing
  all warnings emitted *by* a particular dependency prior to upgrading it
  (the third-party ``warn`` module [3_] does provide this, but enabling it
  involves monkeypatching the standard library's ``warnings`` module).
* re-enabling deprecation warnings by default in __main__ doesn't help in
  handling cases where software has been factored out into support modules, but
  those modules still have little or no automated test coverage. Near term, the
  best currently available answer is to run such applications with
  ``PYTHONWARNINGS=default::DeprecationWarning`` or
  ``python -W default::DeprecationWarning`` and pay attention to their
  ``stderr`` output. Longer term, this is really a question for researchers
  working on static analysis of Python code: how to reliably find usage of
  deprecated APIs, and how to infer that an API or parameter is deprecated
  based on ``warnings.warn`` calls, without actually running either the code
  providing the API or the code accessing it

While these are real problems with the status quo, they're excluded from
consideration in this PEP because they're going to require more complex
solutions than a single additional entry in the default warnings filter,
and resolving them at least potentially won't require going through the PEP
process.

For anyone interested in pursuing them further, the first two would be
``unittest`` module enhancement requests, the third would be a ``warnings``
module enhancement request, while the last would only require a PEP if
inferring API deprecations from their contents was deemed to be an intractable
code analysis problem, and an explicit function and parameter marker syntax in
annotations was proposed instead.

The CPython reference implementation will also include the following related
changes in 3.7:

* a new ``-X dev`` command line option that combines several developer centric
  settings (including ``-Wd``) into one command line flag:
  https://bugs.python.org/issue32043
* changing the behaviour in debug builds to show more of the warnings that are
  off by default in regular interpeter builds: https://bugs.python.org/issue32088


References
==========

.. [1] stdlib-sig thread proposing the original default filter change
   (https://mail.python.org/pipermail/stdlib-sig/2009-November/000789.html)

.. [2] Python 2.7 notification of the default warnings filter change
   (https://docs.python.org/3/whatsnew/2.7.html#changes-to-the-handling-of-deprecation-warnings)

.. [3] Emitting warnings based on the location of the warning itself
   (https://pypi.org/project/warn/)

.. [4] GitHub PR for PEP 565 implementation
   (https://github.com/python/cpython/pull/4458)

.. [5] Tracker issue for PEP 565 implementation
   (https://bugs.python.org/issue31975)

.. [6] python-dev discussion thread for this PEP
   (https://mail.python.org/pipermail/python-dev/2017-November/150477.html)


Copyright
=========

This document has been placed in the public domain.



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