[New-bugs-announce] [issue42470] DeprecationWarning triggers for sequences which happen to be sets as well

[Xavier Morel]

New submission from Xavier Morel <xavier.morel at masklinn.net>:

In 3.9, using `random.sample` on sets triggers

    DeprecationWarning: Sampling from a set deprecated
since Python 3.9 and will be removed in a subsequent version.

*However* it also triggers on types which implement *both* Sequence and Set, despite Sequence on its own being fine.

The issue is that it first checks for Set and triggers a warning, and only then checks that the input is a sequence:

        if isinstance(population, _Set):
            _warn('Sampling from a set deprecated\n'
                  'since Python 3.9 and will be removed in a subsequent version.',
                  DeprecationWarning, 2)
            population = tuple(population)
        if not isinstance(population, _Sequence):
            raise TypeError("Population must be a sequence.  For dicts or sets, use sorted(d).")

the check should rather be:

        if not isinstance(population, _Sequence):
            if isinstance(population, _Set):
                _warn('Sampling from a set deprecated\n'
                      'since Python 3.9 and will be removed in a subsequent version.',
                      DeprecationWarning, 2)
                population = tuple(population)
            else:
                raise TypeError("Population must be a sequence.  For dicts or sets, use sorted(d).")

this also only incurs a single instance check for `_Sequence` types instead of two.

----------
messages: 381885
nosy: xmorel
priority: normal
severity: normal
status: open
title: DeprecationWarning triggers for sequences which happen to be sets as well
versions: Python 3.9

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue42470>
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