BUG: groupby.transform(name) validates name is an aggregation

doc/source/whatsnew/v1.0.0.rst

@@ -78,6 +78,7 @@ Performance improvements
 Bug fixes
 ~~~~~~~~~
 
+- Previously, when :class:`pandas.core.groupby.GroupBy.transform` was passed the name of a transformation (e.g. `rank`, `ffill`, etc) it broadcast the results resulting in erroneous values. Only reduction results and not the results of transformations. The return value in these cases is the same as calling the named function directly (e.g. `g.rank()`) (:issue:`14274`) (:issue:`19354`) (:issue:`22509`).
 
 Categorical
 ^^^^^^^^^^^

pandas/core/groupby/base.py

@@ -144,7 +144,6 @@ def _gotitem(self, key, ndim, subset=None):
     [
         "backfill",
         "bfill",
-        "corrwith",
         "cumcount",
         "cummax",
         "cummin",
@@ -173,6 +172,8 @@ def _gotitem(self, key, ndim, subset=None):
         # are neither a transformation nor a reduction
         "corr",
         "cov",
+        # corrwith does not preserve shape, depending on `other`
+        "corrwith",
         "describe",
         "dtypes",
         "expanding",
@@ -197,4 +198,4 @@ def _gotitem(self, key, ndim, subset=None):
 # Valid values  of `name` for `groupby.transform(name)`
 # NOTE: do NOT edit this directly. New additions should be inserted
 # into the appropriate list above.
-transform_kernel_whitelist = reduction_kernels | transformation_kernels
+groupby_transform_whitelist = reduction_kernels | transformation_kernels

pandas/core/groupby/generic.py

@@ -576,7 +576,7 @@ def transform(self, func, *args, **kwargs):
         func = self._get_cython_func(func) or func
 
         if isinstance(func, str):
-            if not (func in base.transform_kernel_whitelist):
+            if not (func in base.groupby_transform_whitelist):
                 msg = "'{func}' is not a valid function name for transform(name)"
                 raise ValueError(msg.format(func=func))
             if func in base.cythonized_kernels:
@@ -615,7 +615,11 @@ def _transform_fast(self, result, obj, func_nm):
         ids, _, ngroup = self.grouper.group_info
         output = []
         for i, _ in enumerate(result.columns):
-            res = algorithms.take_1d(result.iloc[:, i].values, ids)
+            if func_nm in base.reduction_kernels:
+                # only broadcast results if we performed a reduction
+                res = algorithms.take_1d(result.iloc[:, i]._values, ids)
+            else:
+                res = result.iloc[:, i].values
             if cast:
                 res = self._try_cast(res, obj.iloc[:, i])
             output.append(res)
@@ -1014,7 +1018,7 @@ def transform(self, func, *args, **kwargs):
         func = self._get_cython_func(func) or func
 
         if isinstance(func, str):
-            if not (func in base.transform_kernel_whitelist):
+            if not (func in base.groupby_transform_whitelist):
                 msg = "'{func}' is not a valid function name for transform(name)"
                 raise ValueError(msg.format(func=func))
             if func in base.cythonized_kernels:
@@ -1072,7 +1076,10 @@ def _transform_fast(self, func, func_nm):
 
         ids, _, ngroup = self.grouper.group_info
         cast = self._transform_should_cast(func_nm)
-        out = algorithms.take_1d(func()._values, ids)
+        out = func()
+        if func_nm in base.reduction_kernels:
+            # only broadcast results if we performed a reduction
+            out = algorithms.take_1d(out._values, ids)
         if cast:
             out = self._try_cast(out, self.obj)
         return Series(out, index=self.obj.index, name=self.obj.name)

pandas/tests/groupby/conftest.py

@@ -2,7 +2,7 @@
 import pytest
 
 from pandas import DataFrame, MultiIndex
-from pandas.core.groupby.base import reduction_kernels
+from pandas.core.groupby.base import reduction_kernels, transformation_kernels
 from pandas.util import testing as tm
 
 
@@ -105,6 +105,13 @@ def three_group():
     )
 
 
+@pytest.fixture(params=sorted(transformation_kernels))
+def transformation_func(request):
+    """yields the string names of all groupby reduction functions, one at a time.
+    """
+    return request.param
+
+
 @pytest.fixture(params=sorted(reduction_kernels))
 def reduction_func(request):
     """yields the string names of all groupby reduction functions, one at a time.

pandas/tests/groupby/test_transform.py

@@ -1052,6 +1052,36 @@ def test_transform_agg_by_name(reduction_func, obj):
     assert len(set(DataFrame(result).iloc[-3:, -1])) == 1
 
 
+@pytest.mark.parametrize(
+    "obj",
+    [
+        DataFrame(
+            dict(a=[0, 0, 0, 1, 1, 1], b=range(6)), index=["A", "B", "C", "D", "E", "F"]
+        ),
+        Series([0, 0, 0, 1, 1, 1], index=["A", "B", "C", "D", "E", "F"]),
+    ],
+)
+def test_transform_transformation_by_name(transformation_func, obj):
+    func = transformation_func
+    g = obj.groupby(np.repeat([0, 1], 3))
+
+    if func.startswith("cum"):  # BUG
+        pytest.xfail("skip cum* function tests")
+    if func == "tshift":  # BUG
+        pytest.xfail("tshift")
+
+    args = {"fillna": [0]}.get(func, [])
+
+    result = g.transform(func, *args)
+    # for transformations, g.transform.(name) should return the same result
+    # as `g.name()`
+    expected = getattr(g, func)(*args)
+    if isinstance(obj, DataFrame):
+        tm.assert_frame_equal(result, expected)
+    else:
+        tm.assert_series_equal(result, expected)
+
+
 def test_transform_lambda_with_datetimetz():
     # GH 27496
     df = DataFrame(