title: AttributeError: 'gurobipy.LinExpr' object has no attribute '__colno__'
body: <p>I am trying to model a MILP problem using Python with Gurobi Solver. I have the latest Gurobi solver version. My problem started after I added a constraint with new function of gurobi <strong>m.addGenConstrAbs</strong> which add the abs value of the function as a constraint. Here is my code which create a gurobi feedback as:</p>
  
  <p><strong>AttributeError: 'gurobipy.LinExpr' object has no attribute '__colno__'</strong>.</p>
  
  <p>My code which results with this feedback is:</p>
  
  <pre><code>for t in range(0,Period): 
   m.addGenConstrAbs(PEN[t], EG [t]+STG[t]-XXX, "PEN Constraint") 
  </code></pre>
  
  <p>where <strong>EG[t]</strong>, <strong>STD[t]</strong> and <strong>XXX</strong> are decision variables.</p>
  
  <p>I don't understand why Gurobi or Python returns with this error. What do you think the problem came from? Thanks.</p>