Polish error Info in while_loop (#23183)

* Polish error Info in while_loop test=develop

python/paddle/fluid/layers/control_flow.py

@@ -1031,9 +1031,12 @@ def while_loop(cond, body, loop_vars, is_test=False, name=None):
             output_vars = body(*loop_vars)
         if not isinstance(output_vars, (list, tuple)):
             output_vars = [output_vars]
-        if len(output_vars) != len(loop_vars):
+        try:
+            assert_same_structure(output_vars, loop_vars, check_types=False)
+        except ValueError as e:
             raise ValueError("body in while_loop should return the same arity "
-                             "(length and structure) and types as loop_vars")
+                             "(length and structure) as loop_vars: {0}".format(
+                                 e))
         now_cond = cond(*output_vars)
         map_structure(assign, output_vars, loop_vars)
         assign(now_cond, pre_cond)

python/paddle/fluid/tests/unittests/test_while_loop_op.py

@@ -384,6 +384,23 @@ class TestApiWhileLoop_Error(unittest.TestCase):
         def body_returns_error_type(i, ten):
             return layers.increment(i)
 
+        def cond_returns_with_mutable_dict(i, test_dict):
+            return i > 0
+
+        def body_returns_with_mutable_dict(i, test_dict):
+            test_dict['new_key'] = layers.fill_constant(
+                shape=[1], dtype='int64', value=1)
+            return layers.increment(i), test_dict
+
+        def cond_returns_with_mutable_list(i, test_list):
+            return i > 0
+
+        def body_returns_with_mutable_list(i, test_list):
+            test_list.append(
+                layers.fill_constant(
+                    shape=[1], dtype='int64', value=1))
+            return layers.increment(i), test_list
+
         main_program = Program()
         startup_program = Program()
         with program_guard(main_program, startup_program):
@@ -451,6 +468,31 @@ class TestApiWhileLoop_Error(unittest.TestCase):
 
             self.assertRaises(ValueError, value_error_body_returns_error_type)
 
+            # The length of `output_vars` with mutable value should keep same with `loop_vars`
+            def value_error_body_returns_with_mutable_dict():
+                test_dict = {
+                    "int_constant": layers.fill_constant(
+                        shape=[2, 2], dtype='int64', value=1)
+                }
+                out = layers.while_loop(cond_returns_with_mutable_dict,
+                                        body_returns_with_mutable_dict,
+                                        [data, test_dict])
+
+            self.assertRaises(ValueError,
+                              value_error_body_returns_with_mutable_dict)
+
+            def value_error_body_returns_with_mutable_list():
+                test_list = [
+                    layers.fill_constant(
+                        shape=[2, 2], dtype='int64', value=1)
+                ]
+                out = layers.while_loop(cond_returns_with_mutable_list,
+                                        body_returns_with_mutable_list,
+                                        [data, test_list])
+
+            self.assertRaises(ValueError,
+                              value_error_body_returns_with_mutable_list)
+
 
 if __name__ == '__main__':
     unittest.main()