REF: separate out _get_cython_func_and_vals (pandas-dev#29537)

pandas/core/groupby/ops.py

@@ -393,6 +393,40 @@ def get_func(fname):
 
         return func
 
+    def _get_cython_func_and_vals(
+        self, kind: str, how: str, values: np.ndarray, is_numeric: bool
+    ):
+        """
+        Find the appropriate cython function, casting if necessary.
+
+        Parameters
+        ----------
+        kind : sttr
+        how : srt
+        values : np.ndarray
+        is_numeric : bool
+
+        Returns
+        -------
+        func : callable
+        values : np.ndarray
+        """
+        try:
+            func = self._get_cython_function(kind, how, values, is_numeric)
+        except NotImplementedError:
+            if is_numeric:
+                try:
+                    values = ensure_float64(values)
+                except TypeError:
+                    if lib.infer_dtype(values, skipna=False) == "complex":
+                        values = values.astype(complex)
+                    else:
+                        raise
+                func = self._get_cython_function(kind, how, values, is_numeric)
+            else:
+                raise
+        return func, values
+
     def _cython_operation(
         self, kind: str, values, how: str, axis: int, min_count: int = -1, **kwargs
     ):
@@ -466,20 +500,7 @@ def _cython_operation(
                 )
             out_shape = (self.ngroups,) + values.shape[1:]
 
-        try:
-            func = self._get_cython_function(kind, how, values, is_numeric)
-        except NotImplementedError:
-            if is_numeric:
-                try:
-                    values = ensure_float64(values)
-                except TypeError:
-                    if lib.infer_dtype(values, skipna=False) == "complex":
-                        values = values.astype(complex)
-                    else:
-                        raise
-                func = self._get_cython_function(kind, how, values, is_numeric)
-            else:
-                raise
+        func, values = self._get_cython_func_and_vals(kind, how, values, is_numeric)
 
         if how == "rank":
             out_dtype = "float"