code: def test_reward_function(self):
      string = """
          <pomdpx version="1.0" id="rockSample"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="pomdpx.xsd">
           <Description>RockSample problem for map size 1 x 3.
             Rock is at 0, Rover’s initial position is at 1.
             Exit is at 2.
           </Description>
           <Discount>0.95</Discount>
          <RewardFunction>
          <Func>
              <Var>reward_rover</Var>
              <Parent>action_rover rover_0 rock_0</Parent>
              <Parameter type="DD">
                  <DAG>
                      <Node var="action_rover">
                          <Edge val="amw">
                              <Node var="rover_0">
                                  <Edge val="s0">
                                      <Terminal>-100.0</Terminal>
                                  </Edge>
                                  <Edge val="s1">
                                      <Terminal>0.0</Terminal>
                                  </Edge>
                                  <Edge val="s2">
                                      <Terminal>0.0</Terminal>
                                  </Edge>
                              </Node>
                          </Edge>
                          <Edge val="ame">
                              <Node var="rover_0">
                                  <Edge val="s0">
                                      <Terminal>0.0</Terminal>
                                  </Edge>
                                  <Edge val="s1">
                                      <Terminal>10.0</Terminal>
                                  </Edge>
                                  <Edge val="s2">
                                      <Terminal>0.0</Terminal>
                                  </Edge>
                              </Node>
                          </Edge>
                          <Edge val="ac">
                              <Terminal>0.0</Terminal>
                          </Edge>
                          <Edge val="as">
                              <Node var="rover_0">
                                  <Edge val="s0">
                                      <Node var="rock_0">
                                          <Edge val="good">
                                              <Terminal>10</Terminal>
                                          </Edge>
                                          <Edge val="bad">
                                              <Terminal>-10</Terminal>
                                          </Edge>
                                      </Node>
                                  </Edge>
                                  <Edge val="s1">
                                      <Terminal>-100</Terminal>
                                  </Edge>
                                  <Edge val="s2">
                                      <Terminal>-100</Terminal>
                                  </Edge>
                              </Node>
                          </Edge>
                      </Node>
                  </DAG>
              </Parameter>
          </Func>
      </RewardFunction>
      </pomdpx>
          """
      self.reader_string = PomdpXReader(string=string)
      self.reader_file = PomdpXReader(path=io.StringIO(string))
      expected_reward_function_dd = [
          {
              "Var": "reward_rover",
              "Parent": ["action_rover", "rover_0", "rock_0"],
              "Type": "DD",
              "Parameter": {
                  "action_rover": {
                      "amw": {
                          "rover_0": {
                              "s0": "-100.0",
                              "s1": "0.0",
                              "s2": "0.0",
                          }
                      },
                      "ame": {
                          "rover_0": {
                              "s0": "0.0",
                              "s1": "10.0",
                              "s2": "0.0",
                          }
                      },
                      "ac": "0.0",
                      "as": {
                          "rover_0": {
                              "s0": {
                                  "rock_0": {"good": "10", "bad": "-10"}
                              },
                              "s1": "-100",
                              "s2": "-100",
                          }
                      },
                  }
              },
          }
      ]
      self.maxDiff = None
      self.assertEqual(
          self.reader_string.get_reward_function(),
          expected_reward_function_dd,
      )
      self.assertEqual(
          self.reader_file.get_reward_function(),
          expected_reward_function_dd,
      )
  
imports: import io
  import sys
  import unittest
  import xml.etree.ElementTree
  from pgmpy.readwrite import PomdpXReader, PomdpXWriter
label_desc: Use image node_id 9 for calling the TestPomdpXReaderStringTorch obj's underlying member method code with example usage: obj.test_reward_function() without return types