diff --git a/src/dflow/python/op.py b/src/dflow/python/op.py
index de86150e..ef75d76e 100644
--- a/src/dflow/python/op.py
+++ b/src/dflow/python/op.py
@@ -53,6 +53,7 @@ class OP(ABC):
     slices = {}
     tmp_root = "/tmp"
     create_slice_dir = False
+    pool_size = None
 
     def __init__(
             self,
@@ -417,6 +418,11 @@ def handle_outputs(self, outputs, symlink=False):
             sign = output_sign[name]
             if isinstance(sign, Artifact):
                 slices = self.slices.get(name)
+                if self.pool_size and slices is not None:
+                    if sign.type == str:
+                        sign.type = List[str]
+                    elif sign.type == Path:
+                        sign.type = List[Path]
                 handle_output_artifact(
                     name, outputs[name], sign, slices, self.tmp_root,
                     self.create_slice_dir and slices, symlink=symlink)
diff --git a/src/dflow/python/python_op_template.py b/src/dflow/python/python_op_template.py
index 951195e6..6732c639 100644
--- a/src/dflow/python/python_op_template.py
+++ b/src/dflow/python/python_op_template.py
@@ -604,6 +604,8 @@ def render_script(self):
             else:
                 slices = self.get_slices(output_parameter_slices, name)
                 script += "    op_obj.slices['%s'] = %s\n" % (name, slices)
+        script += "    op_obj.pool_size = %s\n" % getattr(
+            self.slices, "pool_size", None)
 
         script += "    import signal\n"
         script += "    def sigterm_handler(signum, frame):\n"
@@ -671,13 +673,6 @@ def render_script(self):
             for name in sliced_outputs:
                 script += "    output['%s'] = [o.get('%s') if o is not None"\
                     " else None for o in output_list]\n" % (name, name)
-                if isinstance(output_sign[name], Artifact):
-                    if output_sign[name].type == str:
-                        script += "    output_sign['%s'].type = List[str]"\
-                            "\n" % name
-                    elif output_sign[name].type == Path:
-                        script += "    output_sign['%s'].type = List[Path"\
-                            "]\n" % name
         else:
             script += "    try:\n"
             script += "        try:\n"