<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def minimize_l1(self, weights={}):
 """Solve the model by minimizing the L1 norm of the fluxes. If the weights dictionary is given, the weighted L1 norm if minimized instead. The dictionary contains the weights of each reaction (default 1). """


        if self._z is None:
            self._add_minimization_vars()

        objective = self._z.expr(
            (reaction_id, -weights.get(reaction_id, 1))
            for reaction_id in self._model.reactions)
        self._prob.set_objective(objective)

        self._solve()