expose AppendBackward of ProgramDesc to python

paddle/framework/backward.h

@@ -27,6 +27,8 @@ extern std::unique_ptr<OperatorBase> Backward(
     const OperatorBase& forwardOp,
     const std::unordered_set<std::string>& no_grad_vars);
 
+// TODO(someone): Add target as parameter and generate backward op
+// according to target.
 void AppendBackward(ProgramDescBind& program_desc,
                     const std::unordered_set<std::string>& no_grad_vars);
 

paddle/pybind/protobuf.cc

@@ -15,6 +15,7 @@ limitations under the License. */
 #include "paddle/pybind/protobuf.h"
 #include <deque>
 #include <iostream>
+#include "paddle/framework/backward.h"
 #include "paddle/framework/block_desc.h"
 #include "paddle/framework/op_desc.h"
 #include "paddle/framework/program_desc.h"
@@ -116,6 +117,11 @@ void BindProgramDesc(py::module &m) {
                   py::return_value_policy::reference)
       .def("append_block", &ProgramDescBind::AppendBlock,
            py::return_value_policy::reference)
+      .def("backward",
+           [](ProgramDescBind &program_desc,
+              const std::unordered_set<std::string> &no_grad_vars) {
+             AppendBackward(program_desc, no_grad_vars);
+           })
       .def("block", &ProgramDescBind::Block, py::return_value_policy::reference)
       .def("num_blocks", &ProgramDescBind::Size);
 }

python/paddle/v2/framework/tests/test_program.py

@@ -1,4 +1,6 @@
 import unittest
+
+import paddle.v2.framework.core as core
 from paddle.v2.framework.graph import g_program
 
 
@@ -31,6 +33,21 @@ class TestProgram(unittest.TestCase):
         self.assertEqual(1, b.idx)
         self.assertEqual(0, b.parent_idx)
 
+    def test_backward(self):
+        prog = core.ProgramDesc.__create_program_desc__()
+        self.assertIsNotNone(prog)
+        block = prog.block(0)
+        self.assertIsNotNone(block)
+
+        sum_op_desc = block.append_op()
+        sum_op_desc.set_type("sum")
+        sum_op_desc.set_input("X", ["x1", "x2"])
+        sum_op_desc.set_output("Out", ["out"])
+
+        self.assertEqual(len(block.all_ops()), 1)
+        prog.backward(set())
+        self.assertEqual(len(block.all_ops()), 3)
+
 
 if __name__ == '__main__':
     unittest.main()