Why do i get no attribute in docplex?

Question

I am running an optimization code in docplex but for some reason I get an error:

AttributeError: 'NoneType' object has no attribute 'get_value_dict'

This error refers to the line:

prod_hpp_grid_ts = pd.DataFrame.from_dict(sol.get_value_dict(prod_hpp_grid_t), orient='index')

in the code below. Any idea why this is happening?

def sizing_Wind_Solar_Hydrogen(self, prod_wtg_t, prod_pv_t, price_spot):

    globals().update(self.__dict__)
    mdl = Model(name='Sizing')
    time = price_spot.index
    

    prod_hpp_t = mdl.continuous_var_dict(time, lb=0, name='Power produced by HPP')
    .
    .
    .
    '''
    Other variables
    '''

    mdl.maximize(
          -(cost_invest_wtg * cap_wtg)
    .
    .
    .
    '''
    Rest of optimization model
    '''   


    for t in time:
        tt = t + pd.Timedelta('1hour')
        ttt = t - pd.Timedelta('1hour')
        dt = 1

    mdl.add_constraint(prod_hpp_t[t]      == prod_wtg_t[t] * cap_wtg + prod_pv_t[t] * cap_pv )
    .
    .
    .
    '''
    Other constraints
    '''

    sol = mdl.solve(log_output=False)

    prod_hpp_grid_ts = pd.DataFrame.from_dict(sol.get_value_dict(prod_hpp_grid_t), orient='index')
    prod_hpp_grid_ts = prod_hpp_grid_ts.reset_index()   
    P_curt = pd.DataFrame.from_dict(sol.get_value_dict(P_curt), orient='index')
    P_curt = P_curt.reset_index()       
    .
    .
    .
    '''
    Other solutions
    '''

    return sol.get_value(cap_wtg), sol.get_value(cap_pv), prod_hpp_grid_ts[0], \
           P_curt[0], cost_invest_hpp, cost_onm_hpp, LCOE, NPV, IRR

Answer 1

sol is None. This happens when the solve method cannot find a feasible solution. See the documentation: http://ibmdecisionoptimization.github.io/docplex-doc/mp/docplex.mp.model.html#docplex.mp.model.Model.solve