This test:

    def test_counter_data(self):
        # Test that a Counter is treated like any other iterable.
        data = collections.Counter([1, 1, 1, 2])
        # Since the keys of the counter are treated as data points, not the
        # counts, this should return the first mode encountered, 1
        self.assertEqual(self.func(data), 1)

If the mode() code *were* wrong this way (used Counter(data) instead of Counter(iter(data))), then the test wouldn't detect it, as mode() would still return 1. The test data should be [1, 2, 2, 2] instead, in which case such wrong mode() would return 2.

It used to be correct but wasn't adjusted correctly when mode() switched from raising an error for multiple modes to returning the first. The old code was:

    def test_counter_data(self):
        # Test that a Counter is treated like any other iterable.
        data = collections.Counter([1, 1, 1, 2])
        # Since the keys of the counter are treated as data points, not the
        # counts, this should raise.
        self.assertRaises(statistics.StatisticsError, self.func, data)

Thanks for noticing this.

New changeset 48744db70ed519c1566c22bf123a0e1f5c69253f by Raymond Hettinger in branch 'main':
bpo-45852:  Fix the Counter/iter test for statistics.mode() (GH-29667)
https://github.com/python/cpython/commit/48744db70ed519c1566c22bf123a0e1f5c69253f

New changeset 9841ac2da5689ff765250c1abdbf5af9d3750519 by Miss Islington (bot) in branch '3.10':
bpo-45852:  Fix the Counter/iter test for statistics.mode() (GH-29667) (GH-29671)
https://github.com/python/cpython/commit/9841ac2da5689ff765250c1abdbf5af9d3750519