Gurobi error : unable to retrieve attribute 'Xn'

[Hamdy]

I have a facility location problem and I need to ge the best two solutions so in my  code I am utilizing the .xn attribute to make my model faster. it worked originally but for particular instance I get the following error. GurobiError: Unable to retrieve attribute 'Xn' 

even though clearly I have a second to optimal sol

I am using python to code the problem and (pandas to store my variables) 

is there a try catch function that can handle this issue or should i just stop using attribute xn ?

here is part of my code that includes the problem  

m.setParam('SolutionNumber',1)

#%%

Totalsol=[] 

listofsol =[ i for i in range(NumberOfSol)]

for l in listofsol:

#    m.setParam('TimeLimit', 5*60)    

#    m.setParam('MIPGap',0.02)

    

    m.update()

    m.optimize()

    if m.status == GRB.Status.OPTIMAL:

        print('Optimal objective: %g' % m.objVal)

    elif m.status == GRB.Status.INF_OR_UNBD:

        print('Model is infeasible or unbounded')

        exit(0)

    elif m.status == GRB.Status.INFEASIBLE:

        print('Model is infeasible')

        exit(0)

        break

    elif m.status == GRB.Status.UNBOUNDED:

        print('Model is unbounded')

        exit(0)

    else:

        print('Optimization ended with status %d' % m.status)

        exit(0)

 #%%       

    

    FinalStationslist =[]

    VolumPerStationlist = []

    

    useprevsol=False

    listofnewsolutions=[]

    for j in y.axes[0]:

        if y.loc[j,'YVariables'].x>0 :

            FinalStationslist.append(j)

            VolumPerStationlist.append(D.loc[j].x)

            print (j)

            if j not in oldStationsdf.axes[0] and (l< len(listofsol)-1):

                m.addConstr(y.loc[j,'YVariables'] == 0)

                Totalsol.append([j,m.objVal])

                if y.loc[j,'YVariables'].xn==0:

                    useprevsol=True

                else:

                    useprevsol= False

[Renan Garcia]

You should verify that the SolCount model attribute (see https://www.gurobi.com/documentation/6.5/refman/solcount.html#attr:SolCount) has value of at least 2 before querying for the second best solution.

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[Hamdy]

Thank you Very much.