BUG: GroupBy.quantile implicitly sorts index.levels

doc/source/whatsnew/v2.1.0.rst

@@ -412,6 +412,7 @@ Groupby/resample/rolling
   the function operated on the whole index rather than each element of the index. (:issue:`51979`)
 - Bug in :meth:`DataFrameGroupBy.apply` causing an error to be raised when the input :class:`DataFrame` was subset as a :class:`DataFrame` after groupby (``[['a']]`` and not ``['a']``) and the given callable returned :class:`Series` that were not all indexed the same. (:issue:`52444`)
 - Bug in :meth:`GroupBy.groups` with a datetime key in conjunction with another key produced incorrect number of group keys (:issue:`51158`)
+- Bug in :meth:`GroupBy.quantile` may implicitly sort the result index with ``sort=False`` (:issue:`53009`)
 - Bug in :meth:`GroupBy.var` failing to raise ``TypeError`` when called with datetime64, timedelta64 or :class:`PeriodDtype` values (:issue:`52128`, :issue:`53045`)
 -
 

pandas/core/groupby/groupby.py

@@ -70,7 +70,10 @@ class providing the base-class of operations.
 )
 from pandas.util._exceptions import find_stack_level
 
-from pandas.core.dtypes.cast import ensure_dtype_can_hold_na
+from pandas.core.dtypes.cast import (
+    coerce_indexer_dtype,
+    ensure_dtype_can_hold_na,
+)
 from pandas.core.dtypes.common import (
     is_bool_dtype,
     is_float_dtype,
@@ -4309,13 +4312,19 @@ def _insert_quantile_level(idx: Index, qs: npt.NDArray[np.float64]) -> MultiInde
     MultiIndex
     """
     nqs = len(qs)
+    lev_codes, lev = Index(qs).factorize()
+    lev_codes = coerce_indexer_dtype(lev_codes, lev)
 
     if idx._is_multi:
         idx = cast(MultiIndex, idx)
-        lev_codes, lev = Index(qs).factorize()
         levels = list(idx.levels) + [lev]
         codes = [np.repeat(x, nqs) for x in idx.codes] + [np.tile(lev_codes, len(idx))]
         mi = MultiIndex(levels=levels, codes=codes, names=idx.names + [None])
     else:
-        mi = MultiIndex.from_product([idx, qs])
+        nidx = len(idx)
+        idx_codes = coerce_indexer_dtype(np.arange(nidx), idx)
+        levels = [idx, lev]
+        codes = [np.repeat(idx_codes, nqs), np.tile(lev_codes, nidx)]
+        mi = MultiIndex(levels=levels, codes=codes, names=[idx.name, None])
+
     return mi

pandas/tests/groupby/test_quantile.py

@@ -471,3 +471,33 @@ def test_groupby_quantile_dt64tz_period():
     expected.index = expected.index.astype(np.int_)
 
     tm.assert_frame_equal(result, expected)
+
+
+def test_groupby_quantile_nonmulti_levels_order():
+    # Non-regression test for GH #53009
+    ind = pd.MultiIndex.from_tuples(
+        [
+            (0, "a", "B"),
+            (0, "a", "A"),
+            (0, "b", "B"),
+            (0, "b", "A"),
+            (1, "a", "B"),
+            (1, "a", "A"),
+            (1, "b", "B"),
+            (1, "b", "A"),
+        ],
+        names=["sample", "cat0", "cat1"],
+    )
+    ser = pd.Series(range(8), index=ind)
+    result = ser.groupby(level="cat1", sort=False).quantile([0.2, 0.8])
+
+    qind = pd.MultiIndex.from_tuples(
+        [("B", 0.2), ("B", 0.8), ("A", 0.2), ("A", 0.8)], names=["cat1", None]
+    )
+    expected = pd.Series([1.2, 4.8, 2.2, 5.8], index=qind)
+
+    tm.assert_series_equal(result, expected)
+
+    # We need to check that index levels are not sorted
+    expected_levels = pd.core.indexes.frozen.FrozenList([["B", "A"], [0.2, 0.8]])
+    tm.assert_equal(result.index.levels, expected_levels)