<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def species_string(self):
 """ String representation of species on the site. """

        if self.is_ordered:
            return list(self.species.keys())[0].__str__()
        else:
            sorted_species = sorted(self.species.keys())
            return ", ".join(["{}:{:.3f}".format(sp, self.species[sp])
                              for sp in sorted_species])