Fix issue, add test

Author: juleek

doc/source/whatsnew/v2.0.0.rst

@@ -1371,6 +1371,7 @@ Groupby/resample/rolling
 - Bug in :meth:`.DataFrameGroupBy.agg` with ``engine="numba"`` failing to respect ``as_index=False`` (:issue:`51228`)
 - Bug in :meth:`DataFrameGroupBy.agg`, :meth:`SeriesGroupBy.agg`, and :meth:`Resampler.agg` would ignore arguments when passed a list of functions (:issue:`50863`)
 - Bug in :meth:`DataFrameGroupBy.ohlc` ignoring ``as_index=False`` (:issue:`51413`)
+- Bug in :meth:`GroupBy.groups` with a datetime key in conjunction with another key produced incorrect number of group keys (:issue:`51158`)
 
 Reshaping
 ^^^^^^^^^

pandas/core/groupby/ops.py

@@ -1085,6 +1085,9 @@ def groups(self):
  }
  return result
 
+ def __iter__(self) -> Iterator[Hashable]:
+ return iter(self.groupings[0].grouping_vector)
+
  @property
  def nkeys(self) -> int:
  # still matches len(self.groupings), but we can hard-code

pandas/tests/groupby/test_grouping.py

@@ -442,6 +442,29 @@ def test_groupby_grouper_f_sanity_checked(self):
  with pytest.raises(AssertionError, match=msg):
  ts.groupby(lambda key: key[0:6])
 
+ def test_groupby_with_datetime_key(self):
+ # GH 51158
+ df = DataFrame(
+ {
+ "id": ["a", "b"] * 3,
+ "b": date_range("2000-01-01", "2000-01-03", freq="9H"),
+ }
+ )
+ grouper = Grouper(key="b", freq="D")
+ gb = df.groupby([grouper, "id"])
+
+ # test number of groups
+ expected = {
+ (Timestamp("2000-01-01"), "a"): [0, 2],
+ (Timestamp("2000-01-01"), "b"): [1],
+ (Timestamp("2000-01-02"), "a"): [4],
+ (Timestamp("2000-01-02"), "b"): [3, 5],
+ }
+ tm.assert_dict_equal(gb.groups, expected)
+
+ # test number of group keys
+ assert len(gb.groups.keys()) == 4
+
  def test_grouping_error_on_multidim_input(self, df):
  msg = "Grouper for '<class 'pandas.core.frame.DataFrame'>' not 1-dimensional"
  with pytest.raises(ValueError, match=msg):