'Python : Pyomo: problem contrainst 'Cannot convert non-constant expression to bool."

I have a problem on the following form:

**model.obj= Objective(
        expr= sum([ cost_fonction( model.Q[((i1,j1),t)],  for (i1,j1) in Routes_reel for t in time])   ,sense=minimize)**

I would like add a conditionnal constrainst as like : model.reservoir_SM= Var(model.months,domain=NonNegativeReals,doc="Optimized stocks") model.constraint_Saint_Michel= ConstraintList( )

for t in time:
    if t<6: 
        model.constraint_Saint_Michel.add(
            model.reservoir_SM[t] == 0
            )
    elif t==6: 
        model.constraint_Saint_Michel.add(
        model.reservoir_SM[t] == Initial_Stock 
            )
    else  : 
          model.constraint_Saint_Michel.add(
          model.reservoir_SM[t] == model.reservoir_SM[t - 1] -np.maximum(0,(sum([model.Q[((i,j),t)] for (i,j) in Routes_SMA ]) - Vol_prelevable[t-1]))
                                       )

But i receive this error message : Cannot convert non-constant expression to bool. This error is usually caused by using an expression in a boolean context such as an if statement. For example, m.x = Var() if m.x <= 0: ... would cause this exception.

I try to force by using model.Q[((i,j),t)].value. The model solves but the stock incrementation does not work correctly Do you have any idea ?

python pyomo

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'Python : Pyomo: problem contrainst 'Cannot convert non-constant expression to bool."

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