code: def get_length(self, n1, n2, bond_type=BOND_SINGLE):
          
          dataset = self.lengths.get(bond_type)
          if dataset == None:
              return None
          return dataset.get(frozenset([n1, n2]))
docs: Return the length of a bond between n1 and n2 of type bond_type
  
             Arguments:
              | ``n1``  --  the atom number of the first atom in the bond
              | ``n2``  --  the atom number of the second atom the bond
  
             Optional argument:
              | ``bond_type``  --  the type of bond [default=BOND_SINGLE]
  
             This is a safe method for querying a bond_length. If no answer can be
             found, this get_length returns None.