REF: consolidate result-casting in _cython_operation (#38237)

Author: jbrockmendel

pandas/core/groupby/groupby.py

@@ -1000,9 +1000,6 @@ def _cython_transform(
             except NotImplementedError:
                 continue
 
-            if self._transform_should_cast(how):
-                result = maybe_cast_result(result, obj, how=how)
-
             key = base.OutputKey(label=name, position=idx)
             output[key] = result
 
@@ -1081,12 +1078,12 @@ def _cython_agg_general(
                 assert len(agg_names) == result.shape[1]
                 for result_column, result_name in zip(result.T, agg_names):
                     key = base.OutputKey(label=result_name, position=idx)
-                    output[key] = maybe_cast_result(result_column, obj, how=how)
+                    output[key] = result_column
                     idx += 1
             else:
                 assert result.ndim == 1
                 key = base.OutputKey(label=name, position=idx)
-                output[key] = maybe_cast_result(result, obj, how=how)
+                output[key] = result
                 idx += 1
 
         if not output:

pandas/core/groupby/ops.py

@@ -28,7 +28,11 @@
 from pandas.errors import AbstractMethodError
 from pandas.util._decorators import cache_readonly
 
-from pandas.core.dtypes.cast import maybe_cast_result
+from pandas.core.dtypes.cast import (
+    maybe_cast_result,
+    maybe_cast_result_dtype,
+    maybe_downcast_to_dtype,
+)
 from pandas.core.dtypes.common import (
     ensure_float,
     ensure_float64,
@@ -620,8 +624,11 @@ def _cython_operation(
         if swapped:
             result = result.swapaxes(0, axis)
 
-        if is_datetimelike and kind == "aggregate":
-            result = result.astype(orig_values.dtype)
+        if how not in base.cython_cast_blocklist:
+            # e.g. if we are int64 and need to restore to datetime64/timedelta64
+            # "rank" is the only member of cython_cast_blocklist we get here
+            dtype = maybe_cast_result_dtype(orig_values.dtype, how)
+            result = maybe_downcast_to_dtype(result, dtype)
 
         return result, names