new_contents: # Copyright (c) 2018 PaddlePaddle Authors. All Rights Reserved.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  
  from __future__ import print_function
  
  import six
  
  from collections import defaultdict
  from paddle.fluid import core
  from paddle.fluid import framework
  
  __all__ = ['Tracer']
  
  
  def release_op(op):
      del framework._imperative_tracer()._ops[op._trace_id]
  
  
  class Tracer(core.Tracer):
      """
      Python wrapper of imperative tracer
      """
  
      def __init__(self, block):
          super(Tracer, self).__init__(block)
  
          self._ops = defaultdict()
          self._trace_id = 0
  
      def trace_op(self, op, stop_gradient=False):
          # record op's trace id
          op.iop._trace_id = self._trace_id
          self._trace_id += 1
  
          # trace op and save it
          backward_refs = self.trace(op.iop, op.inputs, op.outputs, op.block.desc,
                                     framework._current_expected_place(),
                                     stop_gradient)
  
          if not stop_gradient:
              self._ops[op.iop._trace_id] = op
  
              # register backward hooks and variables if needed
              if len(backward_refs) > 0:
                  op.iop.register_backward_hooks(release_op)
  
                  op.backward_refs = defaultdict(list)
                  for k, v in six.iteritems(op.inputs):
                      if k in backward_refs:
                          op.backward_refs[k] = op.inputs[k]
  
                  for k, v in six.iteritems(op.outputs):
                      if k in backward_refs:
                          op.backward_refs[k] = op.outputs[k]
  
subject: Add tracer implementation