bpo-39674: Suggest to test with DeprecationWarning (GH-18552)
Add a section in What's New In Python 3.9 to strongly advice to check for DeprecationWarning in your Python projects. Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>
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@ -66,6 +66,34 @@ Summary -- Release highlights
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.. PEP-sized items next.
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You should check for DeprecationWarning in your code
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====================================================
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When Python 2.7 was still supported, many functions were kept for backward
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compatibility with Python 2.7. With the end of Python 2.7 support, these
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backward compatibility layers have been removed, or will be removed soon.
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Most of them emitted a :exc:`DeprecationWarning` warning for several years. For
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example, using ``collections.Mapping`` instead of ``collections.abc.Mapping``
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emits a :exc:`DeprecationWarning` since Python 3.3, released in 2012.
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Test your application with the :option:`-W` ``default`` command-line option to see
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:exc:`DeprecationWarning` and :exc:`PendingDeprecationWarning`, or even with
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:option:`-W` ``error`` to treat them as errors. :ref:`Warnings Filter
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<warning-filter>` can be used to ignore warnings from third-party code.
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It has been decided to keep a few backward compatibility layers for one last
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release, to give more time to Python projects maintainers to organize the
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removal of the Python 2 support and add support for Python 3.9.
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Aliases to ref:`Abstract Base Classes <collections-abstract-base-classes>` in
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the :mod:`collections` module, like ``collections.Mapping`` alias to
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:class:`collections.abc.Mapping`, are kept for one last release for backward
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compatibility. They will be removed from Python 3.10.
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More generally, try to run your tests in the :ref:`Python Development Mode
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<devmode>` which helps to prepare your code to make it compatible with the
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next Python version.
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New Features
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============
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