ENH: Groupby.transform support string input with engine=numba (pandas-dev#53579)

* ENH: Groupby.transform support string input with engine=numba

* remove xfail

Author: lithomas1

doc/source/whatsnew/v2.1.0.rst

@@ -103,6 +103,7 @@ Other enhancements
 - :meth:`DataFrame.unstack` gained the ``sort`` keyword to dictate whether the resulting :class:`MultiIndex` levels are sorted (:issue:`15105`)
 - :meth:`DataFrameGroupby.agg` and :meth:`DataFrameGroupby.transform` now support grouping by multiple keys when the index is not a :class:`MultiIndex` for ``engine="numba"`` (:issue:`53486`)
 - :meth:`SeriesGroupby.agg` and :meth:`DataFrameGroupby.agg` now support passing in multiple functions for ``engine="numba"`` (:issue:`53486`)
+- :meth:`SeriesGroupby.transform` and :meth:`DataFrameGroupby.transform` now support passing in a string as the function for ``engine="numba"`` (:issue:`53579`)
 - Added ``engine_kwargs`` parameter to :meth:`DataFrame.to_excel` (:issue:`53220`)
 - Added a new parameter ``by_row`` to :meth:`Series.apply`. When set to ``False`` the supplied callables will always operate on the whole Series (:issue:`53400`).
 - Many read/to_* functions, such as :meth:`DataFrame.to_pickle` and :func:`read_csv`, support forwarding compression arguments to lzma.LZMAFile (:issue:`52979`)

pandas/core/groupby/generic.py

@@ -523,10 +523,16 @@ def _cython_transform(
 
         return obj._constructor(result, index=self.obj.index, name=obj.name)
 
-    def _transform_general(self, func: Callable, *args, **kwargs) -> Series:
+    def _transform_general(
+        self, func: Callable, engine, engine_kwargs, *args, **kwargs
+    ) -> Series:
         """
         Transform with a callable `func`.
         """
+        if maybe_use_numba(engine):
+            return self._transform_with_numba(
+                func, *args, engine_kwargs=engine_kwargs, **kwargs
+            )
         assert callable(func)
         klass = type(self.obj)
 
@@ -1654,7 +1660,11 @@ def arr_func(bvalues: ArrayLike) -> ArrayLike:
         res_df = self._maybe_transpose_result(res_df)
         return res_df
 
-    def _transform_general(self, func, *args, **kwargs):
+    def _transform_general(self, func, engine, engine_kwargs, *args, **kwargs):
+        if maybe_use_numba(engine):
+            return self._transform_with_numba(
+                func, *args, engine_kwargs=engine_kwargs, **kwargs
+            )
         from pandas.core.reshape.concat import concat
 
         applied = []

pandas/core/groupby/groupby.py

@@ -1787,22 +1787,20 @@ def _cython_transform(
 
     @final
     def _transform(self, func, *args, engine=None, engine_kwargs=None, **kwargs):
-        if maybe_use_numba(engine):
-            return self._transform_with_numba(
-                func, *args, engine_kwargs=engine_kwargs, **kwargs
-            )
-
         # optimized transforms
         func = com.get_cython_func(func) or func
 
         if not isinstance(func, str):
-            return self._transform_general(func, *args, **kwargs)
+            return self._transform_general(func, engine, engine_kwargs, *args, **kwargs)
 
         elif func not in base.transform_kernel_allowlist:
             msg = f"'{func}' is not a valid function name for transform(name)"
             raise ValueError(msg)
         elif func in base.cythonized_kernels or func in base.transformation_kernels:
             # cythonized transform or canned "agg+broadcast"
+            if engine is not None:
+                kwargs["engine"] = engine
+                kwargs["engine_kwargs"] = engine_kwargs
             return getattr(self, func)(*args, **kwargs)
 
         else:
@@ -1817,6 +1815,9 @@ def _transform(self, func, *args, engine=None, engine_kwargs=None, **kwargs):
                 with com.temp_setattr(self, "as_index", True):
                     # GH#49834 - result needs groups in the index for
                     # _wrap_transform_fast_result
+                    if engine is not None:
+                        kwargs["engine"] = engine
+                        kwargs["engine_kwargs"] = engine_kwargs
                     result = getattr(self, func)(*args, **kwargs)
 
             return self._wrap_transform_fast_result(result)

pandas/tests/groupby/transform/test_numba.py

@@ -132,20 +132,25 @@ def func_1(values, index):
     tm.assert_frame_equal(expected, result)
 
 
+# TODO: Test more than just reductions (e.g. actually test transformations once we have
 @td.skip_if_no("numba")
 @pytest.mark.parametrize(
     "agg_func", [["min", "max"], "min", {"B": ["min", "max"], "C": "sum"}]
 )
-def test_multifunc_notimplimented(agg_func):
+def test_string_cython_vs_numba(agg_func, numba_supported_reductions):
+    agg_func, kwargs = numba_supported_reductions
     data = DataFrame(
         {0: ["a", "a", "b", "b", "a"], 1: [1.0, 2.0, 3.0, 4.0, 5.0]}, columns=[0, 1]
     )
     grouped = data.groupby(0)
-    with pytest.raises(NotImplementedError, match="Numba engine can"):
-        grouped.transform(agg_func, engine="numba")
 
-    with pytest.raises(NotImplementedError, match="Numba engine can"):
-        grouped[1].transform(agg_func, engine="numba")
+    result = grouped.transform(agg_func, engine="numba", **kwargs)
+    expected = grouped.transform(agg_func, engine="cython", **kwargs)
+    tm.assert_frame_equal(result, expected)
+
+    result = grouped[1].transform(agg_func, engine="numba", **kwargs)
+    expected = grouped[1].transform(agg_func, engine="cython", **kwargs)
+    tm.assert_series_equal(result, expected)
 
 
 @td.skip_if_no("numba")
@@ -234,9 +239,6 @@ def numba_func(values, index):
 
 
 @td.skip_if_no("numba")
-@pytest.mark.xfail(
-    reason="Groupby transform doesn't support strings as function inputs yet with numba"
-)
 def test_multilabel_numba_vs_cython(numba_supported_reductions):
     reduction, kwargs = numba_supported_reductions
     df = DataFrame(