code: def anumb_to_atom(self, anumb):
          '''Returns the atom object corresponding to an atom number'''
  
          assert isinstance(anumb, int), "anumb must be integer"
  
          if not self._anumb_to_atom:   # empty dictionary
  
              if self.atoms:
                  for atom in self.atoms:
                      self._anumb_to_atom[atom.number] = atom
                  return self._anumb_to_atom[anumb]
              else:
                  self.logger("no atoms in the molecule")
                  return False
  
          else:
              if anumb in self._anumb_to_atom:
                  return self._anumb_to_atom[anumb]
              else:
                  self.logger("no such atom number ({0:d}) in the molecule".format(anumb))
                  return Falsf anumb_to_atom(self, anumb):
          '''Returns the atom object corresponding to an atom number'''
  
          assert isinstance(anumb, int), "anumb must be integer"
  
          if not self._anumb_to_atom:   # empty dictionary
  
              if self.atoms:
                  for atom in self.atoms:
                      self._anumb_to_atom[atom.number] = atom
                  return self._anumb_to_atom[anumb]
              else:
                  self.logger("no atoms in the molecule")
                  return False
  
          else:
              if anumb in self._anumb_to_atom:
                  return self._anumb_to_atom[anumb]
              else:
                  self.logger("no such atom number ({0:d}) in the molecule".format(anumb))
                  return False
docs: Returns the atom object corresponding to an atom number