old_contents: from mdtraj.pdb import element
  from mdtraj.testing import assert_raises
  
  def test_immutable():
      with assert_raises(AttributeError):
          element.hydrogen.mass = 1
      with assert_raises(AttributeError):
          element.radium.symbol = 'sdfsdfsdf'
      with assert_raises(AttributeError):
          element.iron.name = 'sdfsdf'
  
      assert element.hydrogen.mass == 1.007947
      assert element.radium.symbol == 'Ra'
      assert element.iron.name == 'iron'
  
new_contents: from mdtraj.pdb import element
  from mdtraj.testing import assert_raises
  
  def test_immutable():
      def f():
          element.hydrogen.mass = 1
      def g():
          element.radium.symbol = 'sdfsdfsdf'
      def h():
          element.iron.name = 'sdfsdf'
  
      assert_raises(AttributeError, f)
      assert_raises(AttributeError, g)
      assert_raises(AttributeError, h)
      assert element.hydrogen.mass == 1.007947
      assert element.radium.symbol == 'Ra'
      assert element.iron.name == 'iron'
  
subject: fix test_element for python2.6