old_contents: """
  Viewing a reaction path diagram.
  
  This script uses Graphviz to generate an image. You must have Graphviz installed
  and the program 'dot' must be on your path for this example to work.
  Graphviz can be obtained from http://www.graphviz.org/ or (possibly) installed
  using your operating system's package manager.
  """
  
  import os
  import sys
  
  import cantera as ct
  
  # these lines can be replaced by any commands that generate
  # an object of a class derived from class Kinetics in some state.
  gas = ct.Solution('gri30.xml')
  gas.TPX = 1300.0, ct.one_atm, 'CH4:0.4, O2:1, N2:3.76'
  r = ct.Reactor(gas)
  net = ct.ReactorNet([r])
  T = r.T
  while T < 1900:
      net.step(1.0)
      T = r.T
  
  element = 'N'
  
  diagram = ct.ReactionPathDiagram(gas, element)
  diagram.title = 'Reaction path diagram following {0}'.format(element)
  diagram.label_threshold = 0.01
  
  dot_file = 'rxnpath.dot'
  img_file = 'rxnpath.png'
  img_path = os.path.join(os.getcwd(), img_file)
  
  diagram.write_dot(dot_file)
  print diagram.get_data()
  
  print("Wrote graphviz input file to '{0}'.".format(os.path.join(os.getcwd(), dot_file)))
  
  os.system('dot {0} -Tpng -o{1} -Gdpi=200'.format(dot_file, img_file))
  print("Wrote graphviz output file to '{0}'.".format(img_path))
  
  if "-view" in sys.argv:
      import webbrowser
      webbrowser.open('file:///' + img_path)
  
new_contents: """
  Viewing a reaction path diagram.
  
  This script uses Graphviz to generate an image. You must have Graphviz installed
  and the program 'dot' must be on your path for this example to work.
  Graphviz can be obtained from http://www.graphviz.org/ or (possibly) installed
  using your operating system's package manager.
  """
  
  import os
  import sys
  
  import cantera as ct
  
  # these lines can be replaced by any commands that generate
  # an object of a class derived from class Kinetics in some state.
  gas = ct.Solution('gri30.xml')
  gas.TPX = 1300.0, ct.one_atm, 'CH4:0.4, O2:1, N2:3.76'
  r = ct.Reactor(gas)
  net = ct.ReactorNet([r])
  T = r.T
  while T < 1900:
      net.step(1.0)
      T = r.T
  
  element = 'N'
  
  diagram = ct.ReactionPathDiagram(gas, element)
  diagram.title = 'Reaction path diagram following {0}'.format(element)
  diagram.label_threshold = 0.01
  
  dot_file = 'rxnpath.dot'
  img_file = 'rxnpath.png'
  img_path = os.path.join(os.getcwd(), img_file)
  
  diagram.write_dot(dot_file)
  print(diagram.get_data())
  
  print("Wrote graphviz input file to '{0}'.".format(os.path.join(os.getcwd(), dot_file)))
  
  os.system('dot {0} -Tpng -o{1} -Gdpi=200'.format(dot_file, img_file))
  print("Wrote graphviz output file to '{0}'.".format(img_path))
  
  if "-view" in sys.argv:
      import webbrowser
      webbrowser.open('file:///' + img_path)
  
subject: Fix Python 3 compatibility issue