[Dy2Stat] 1. Fix bug of for-range stmts. 2. Support that step value is negative in for-range stmts (#29519)

1. Fix error in _build_cond_stmt of for-range stmts. 

2. Support that step value is negative in for-range stmts

3. Fix code because of the diff between Py2 and Py3

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

@@ -1028,18 +1028,40 @@ class ForNodeVisitor(object):
         return step_node
 
     def _build_cond_stmt(self, step_node, compare_node):
-        return gast.Compare(
-            left=gast.BinOp(
+        if not isinstance(step_node, (gast.Constant, gast.UnaryOp)):
+            raise NotImplementedError(
+                "Dynamic-to-Static only supports the step value is a constant or negative constant in 'for-range' statements, "
+                "such as '2', '-3'. But received: '{}'. Please fix code to be compatible with Dynamic-to-Static."
+                .format(ast_to_source_code(step_node).strip()))
+
+        if isinstance(step_node, gast.UnaryOp) or step_node.value < 0:
+            # eg:
+            # range(max, min, -2)
+            # ->
+            # i > min
+            return gast.Compare(
                 left=gast.Name(
                     id=self.iter_var_name
                     if self.is_for_range_iter() else self.iter_idx_name,
                     ctx=gast.Load(),
                     annotation=None,
                     type_comment=None),
-                op=gast.Add(),
-                right=step_node),
-            ops=[gast.LtE()],
-            comparators=[compare_node])
+                ops=[gast.Gt()],
+                comparators=[compare_node])
+        else:
+            # eg:
+            # range(min, max, 2)
+            # ->
+            # i < max
+            return gast.Compare(
+                left=gast.Name(
+                    id=self.iter_var_name
+                    if self.is_for_range_iter() else self.iter_idx_name,
+                    ctx=gast.Load(),
+                    annotation=None,
+                    type_comment=None),
+                ops=[gast.Lt()],
+                comparators=[compare_node])
 
     def _build_index_increase_node(self, step_node):
         return gast.AugAssign(

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

@@ -94,6 +94,28 @@ def for_loop_dyfunc2(max_len):
     return ret
 
 
+def for_loop_dyfunc3(max_len):
+    ret = fluid.layers.zeros(shape=[1], dtype='float32')
+    for i in range(1, 10, 2):
+        fluid.layers.increment(ret, value=2.0, in_place=True)
+    return ret
+
+
+def for_loop_dyfunc4(max_len):
+    ret = fluid.layers.zeros(shape=[1], dtype='float32')
+    for i in range(10, 1, -2):
+        fluid.layers.increment(ret, value=2.0, in_place=True)
+    return ret
+
+
+def for_loop_dyfunc_not_support(max_len):
+    ret = fluid.layers.zeros(shape=[1], dtype='float32')
+    a = -2
+    for i in range(10, 1, a):
+        fluid.layers.increment(ret, value=2.0, in_place=True)
+    return ret
+
+
 def while_loop_bool_op(x):
     i = fluid.dygraph.to_variable(x)
 
@@ -333,6 +355,16 @@ class TestTransformForLoop2(TestTransformForLoop):
         self.dyfunc = for_loop_dyfunc2
 
 
+class TestTransformForLoop3(TestTransformForLoop):
+    def _init_dyfunc(self):
+        self.dyfunc = for_loop_dyfunc3
+
+
+class TestTransformForLoop4(TestTransformForLoop):
+    def _init_dyfunc(self):
+        self.dyfunc = for_loop_dyfunc4
+
+
 class TestClassVarInForLoop(TestTransformForLoop):
     def _init_dyfunc(self):
         self.dyfunc = for_loop_class_var
@@ -343,5 +375,17 @@ class TestVarCreateInForLoop(TestTransformForLoop):
         self.dyfunc = var_create_in_for_loop
 
 
+class TestErrorInForLoop(TestTransformForLoop):
+    def _init_dyfunc(self):
+        self.dyfunc = for_loop_dyfunc_not_support
+
+    def test_ast_to_func(self):
+        with self.assertRaisesRegexp(
+                NotImplementedError,
+                "Dynamic-to-Static only supports the step value is a constant or negative constant "
+        ):
+            self._run_static()
+
+
 if __name__ == '__main__':
     unittest.main()