API/BUG: freq retention in value_counts #33830
Description
dti = pd.date_range('2016-01-01', periods=5)
dti.value_counts().index.freq # <-- None
dti.factorize()[1].freq # <-- None
mi = pd.MultiIndex.from_arrays([dti, dti])
mi.levels[0].freq # <-- None
There is a comment in tests.indexes.datetimes.test_datetime test_factorize suggesting that freq should be preserved by factorize, but that is not checked and would fail if it were
# freq must be preserved
idx3 = date_range("2000-01", periods=4, freq="M", tz="Asia/Tokyo")
exp_arr = np.array([0, 1, 2, 3], dtype=np.intp)
arr, idx = idx3.factorize()
tm.assert_numpy_array_equal(arr, exp_arr)
tm.assert_index_equal(idx, idx3)
So the question: do we want to try to preserve freq in factorize?
xref #33677 for the MultiIndex case
Update One more: Categorical:
dti = pd.date_range('2016-01-01', periods=5)
cat = pd.Categorical(dti)
cat.categories.freq # <-- None