code: def parse(cls, constraints_expression):
          
          constraint_exprs = re.split(r'\s*,\s*', constraints_expression)
          return Constraints(merge(Constraint.parse(constraint_expr)
                                   for constraint_expr in constraint_exprs))
docs: Parses a :ref:`constraints_expression <constraints-expressions>`
          and returns a :class:`Constraints` object.