def reaction_weight(reaction):
if len(reaction.metabolites) != <NUM_LIT:1>:<EOL><INDENT>raise ValueError('<STR_LIT>'<EOL>'<STR_LIT>')<EOL><DEDENT>met, coeff = next(iteritems(reaction.metabolites))<EOL>return [coeff * met.formula_weight]<EOL>
Return the metabolite weight times its stoichiometric coefficient.