[Dy2Stat]Enhance nonlocal machanism while nonlocal vars is empty (#43848)

python/paddle/fluid/dygraph/dygraph_to_static/ifelse_transformer.py

@@ -524,7 +524,8 @@ def transform_if_else(node, root):
     if ARGS_NAME in nonlocal_names:
         nonlocal_names.remove(ARGS_NAME)
 
-    nonlocal_stmt_node = [create_nonlocal_stmt_node(nonlocal_names)]
+    nonlocal_stmt_node = [create_nonlocal_stmt_node(nonlocal_names)
+                          ] if nonlocal_names else []
 
     empty_arg_node = gast.arguments(args=[],
                                     posonlyargs=[],
@@ -557,8 +558,20 @@ def create_get_args_node(names):
 
         def get_args_0():
             nonlocal x, y
+            return x, y
     """
+
+    def empty_node():
+        func_def = """
+        def {func_name}():
+            return
+        """.format(func_name=unique_name.generate(GET_ARGS_FUNC_PREFIX))
+        return gast.parse(textwrap.dedent(func_def)).body[0]
+
     assert isinstance(names, (list, tuple))
+    if not names:
+        return empty_node()
+
     template = """
     def {func_name}():
         nonlocal {vars}
@@ -578,7 +591,19 @@ def create_set_args_node(names):
             nonlocal x, y
             x, y = __args
     """
+
+    def empty_node():
+        func_def = """
+        def {func_name}({args}):
+            pass
+        """.format(func_name=unique_name.generate(SET_ARGS_FUNC_PREFIX),
+                   args=ARGS_NAME)
+        return gast.parse(textwrap.dedent(func_def)).body[0]
+
     assert isinstance(names, (list, tuple))
+    if not names:
+        return empty_node()
+
     template = """
     def {func_name}({args}):
         nonlocal {vars}

python/paddle/fluid/tests/unittests/dygraph_to_static/ifelse_simple_func.py

@@ -28,6 +28,18 @@ def loss_fn(x, lable):
     return loss
 
 
+def dyfunc_empty_nonlocal(x):
+    flag = True
+    if flag:
+        print("It's a test for empty nonlocal stmt")
+
+    if paddle.mean(x) < 0:
+        x + 1
+
+    out = x * 2
+    return out
+
+
 def dyfunc_with_if_else(x_v, label=None):
     if fluid.layers.mean(x_v).numpy()[0] > 5:
         x_v = x_v - 1

python/paddle/fluid/tests/unittests/dygraph_to_static/test_ifelse.py

@@ -73,6 +73,13 @@ class TestDygraphIfElse3(TestDygraphIfElse):
         self.dyfunc = dyfunc_with_if_else3
 
 
+class TestDygraphIfElse4(TestDygraphIfElse):
+
+    def setUp(self):
+        self.x = np.random.random([10, 16]).astype('float32')
+        self.dyfunc = dyfunc_empty_nonlocal
+
+
 class TestDygraphIfElseWithListGenerator(TestDygraphIfElse):
 
     def setUp(self):