BUG: GH16875

Fix inconsistency in groupby trnaformations

Author: jeetjitsu

doc/source/whatsnew/v0.21.0.txt

@@ -173,7 +173,7 @@ Groupby/Resample/Rolling
 - Bug in ``DataFrame.resample().size()`` where an empty ``DataFrame`` did not return a ``Series`` (:issue:`14962`)
 - Bug in ``infer_freq`` causing indices with 2-day gaps during the working week to be wrongly inferred as business daily (:issue:`16624`)
 - Bug in ``.rolling.quantile()`` which incorrectly used different defaults than :func:`Series.quantile()` and :func:`DataFrame.quantile()` (:issue:`9413`, :issue:`16211`)
-
+- Bug in ``groupby.transform()`` that would coerce the boolean dtypes back to float (:issue:`16875`)
 
 Sparse
 ^^^^^^

pandas/core/groupby.py

@@ -3055,7 +3055,7 @@ def transform(self, func, *args, **kwargs):
         # we have a numeric dtype, as these are *always* udfs
         # the cython take a different path (and casting)
         dtype = self._selected_obj.dtype
-        if is_numeric_dtype(dtype):
+        if is_numeric_dtype(dtype) and not is_bool_dtype(result.dtype):
             result = maybe_downcast_to_dtype(result, dtype)
 
         result.name = self._selected_obj.name

pandas/tests/groupby/test_transform.py

@@ -195,6 +195,19 @@ def test_transform_bug(self):
         expected = Series(np.arange(5, 0, step=-1), name='B')
         assert_series_equal(result, expected)
 
+    def test_transform_numeric_to_boolean(self):
+        # GH 16875
+        # inconsistency in transforming boolean values
+        expected = pd.Series([True, True], name='A')
+
+        df = pd.DataFrame({'A': [1.1, 2.2], 'B': [1, 2]})
+        result = df.groupby('B').A.transform(lambda x: True)
+        assert_series_equal(result, expected)
+
+        df = pd.DataFrame({'A': [1, 2], 'B': [1, 2]})
+        result = df.groupby('B').A.transform(lambda x: True)
+        assert_series_equal(result, expected)
+
     def test_transform_datetime_to_timedelta(self):
         # GH 15429
         # transforming a datetime to timedelta