old_contents: class RectangularMesh(object):
      def __init__(self, d, atlas='atlas', meshname='mesh'):
          if not isinstance(d, (tuple, list)) or len(d) != 3:
              raise ValueError('Cellsize d must be a tuple of length 3.')
          elif d[0] <= 0 or d[1] <= 0 or d[2] <= 0:
              raise ValueError('Cellsize dimensions must be positive.')
          else:
              self.d = d
  
          if not isinstance(atlas, str):
              raise ValueError('atlas must be a string.')
          else:
              self.atlas = atlas
  
          if not isinstance(meshname, str):
              raise ValueError('name must be a string.')
          else:
              self.meshname = meshname
  
      def get_mif(self):
          data = [('xstep', self.d[0]),
                  ('ystep', self.d[1]),
                  ('zstep', self.d[2])]
  
          # Create mif string.
          mif = '# RectangularMesh\n'
          for datum in data:
              mif += 'set {} {}\n'.format(datum[0], datum[1])
          mif += 'Specify Oxs_RectangularMesh:{}'.format(self.meshname) + ' {\n'
          mif += '\tcellsize { $xstep $ystep $zstep }\n'
          mif += '\tatlas {}\n'.format(self.atlas)
          mif += '}\n\n'
  
          return mif
  
new_contents: class RectangularMesh(object):
      def __init__(self, d, atlas='atlas', meshname='mesh'):
          if not isinstance(d, (tuple, list)) or len(d) != 3:
              raise ValueError('Cellsize d must be a tuple of length 3.')
          elif d[0] <= 0 or d[1] <= 0 or d[2] <= 0:
              raise ValueError('Cellsize dimensions must be positive.')
          else:
              self.d = d
  
          if not isinstance(atlas, str):
              raise ValueError('atlas must be a string.')
          else:
              self.atlas = atlas
  
          if not isinstance(meshname, str):
              raise ValueError('name must be a string.')
          else:
              self.meshname = meshname
  
      def get_mif(self):
          # Create mif string.
          mif = '# RectangularMesh\n'
          mif += 'Specify Oxs_RectangularMesh:{}'.format(self.meshname) + ' {\n'
          mif += '\tcellsize {'
          mif += ' {} {} {} '.format(self.d[0], self.d[1], self.d[2])
          mif += '}\n'
          mif += '\tatlas {}\n'.format(self.atlas)
          mif += '}\n\n'
  
          return mif
  
subject: Remove set statements.