BUG: Groupby.agg cannot reduce with tz aware data

pandas/_libs/reduction.pyx

@@ -342,7 +342,8 @@ cdef class SeriesGrouper:
  index = None
  else:
  values = dummy.values
- if dummy.dtype != self.arr.dtype:
+ if (dummy.dtype != self.arr.dtype
+ and values.dtype != self.arr.dtype):
  raise ValueError('Dummy array must be same dtype')
  if not values.flags.contiguous:
  values = values.copy()

pandas/tests/groupby/aggregate/test_other.py

@@ -512,3 +512,18 @@ def test_agg_list_like_func():
  expected = pd.DataFrame({'A': [str(x) for x in range(3)],
  'B': [[str(x)] for x in range(3)]})
  tm.assert_frame_equal(result, expected)
+
+
+def test_agg_lambda_with_timezone():
+ # GH 23683
+ df = pd.DataFrame({
+ 'tag': [1, 1],
+ 'date': [
+ pd.Timestamp('2018-01-01', tz='UTC'),
+ pd.Timestamp('2018-01-02', tz='UTC')]
+ })
+ result = df.groupby('tag').agg({'date': lambda e: e.head(1)})
+ expected = pd.DataFrame([pd.Timestamp('2018-01-01', tz='UTC')],
+ index=pd.Index([1], name='tag'),
+ columns=['date'])
+ tm.assert_frame_equal(result, expected)