REF: _cython_operation handle values.ndim==1 case up-front (#40672)

pandas/core/groupby/ops.py

@@ -609,6 +609,23 @@ def _cython_operation(
                 kind, values, how, axis, min_count, **kwargs
             )
 
+        elif values.ndim == 1:
+            # expand to 2d, dispatch, then squeeze if appropriate
+            values2d = values[None, :]
+            res = self._cython_operation(
+                kind=kind,
+                values=values2d,
+                how=how,
+                axis=1,
+                min_count=min_count,
+                **kwargs,
+            )
+            if res.shape[0] == 1:
+                return res[0]
+
+            # otherwise we have OHLC
+            return res.T
+
         is_datetimelike = needs_i8_conversion(dtype)
 
         if is_datetimelike:
@@ -629,22 +646,20 @@ def _cython_operation(
             values = values.astype(object)
 
         arity = self._cython_arity.get(how, 1)
+        ngroups = self.ngroups
 
-        vdim = values.ndim
-        swapped = False
-        if vdim == 1:
-            values = values[:, None]
-            out_shape = (self.ngroups, arity)
+        assert axis == 1
+        values = values.T
+        if how == "ohlc":
+            out_shape = (ngroups, 4)
+        elif arity > 1:
+            raise NotImplementedError(
+                "arity of more than 1 is not supported for the 'how' argument"
+            )
+        elif kind == "transform":
+            out_shape = values.shape
         else:
-            if axis > 0:
-                swapped = True
-                assert axis == 1, axis
-                values = values.T
-            if arity > 1:
-                raise NotImplementedError(
-                    "arity of more than 1 is not supported for the 'how' argument"
-                )
-            out_shape = (self.ngroups,) + values.shape[1:]
+            out_shape = (ngroups,) + values.shape[1:]
 
         func, values = self._get_cython_func_and_vals(kind, how, values, is_numeric)
 
@@ -658,13 +673,11 @@ def _cython_operation(
 
         codes, _, _ = self.group_info
 
+        result = maybe_fill(np.empty(out_shape, dtype=out_dtype))
         if kind == "aggregate":
-            result = maybe_fill(np.empty(out_shape, dtype=out_dtype))
             counts = np.zeros(self.ngroups, dtype=np.int64)
             result = self._aggregate(result, counts, values, codes, func, min_count)
         elif kind == "transform":
-            result = maybe_fill(np.empty(values.shape, dtype=out_dtype))
-
             # TODO: min_count
             result = self._transform(
                 result, values, codes, func, is_datetimelike, **kwargs
@@ -680,11 +693,7 @@ def _cython_operation(
             assert result.ndim != 2
             result = result[counts > 0]
 
-        if vdim == 1 and arity == 1:
-            result = result[:, 0]
-
-        if swapped:
-            result = result.swapaxes(0, axis)
+        result = result.T
 
         if how not in base.cython_cast_blocklist:
             # e.g. if we are int64 and need to restore to datetime64/timedelta64