title: Retrieving the linear expression value
body: <p>I would like to know how can I retrieve the linear expression value to show in my result. In my problem, I have the following expression:</p>
  
  <pre><code>total_cost = solver.Sum(supplier_cost[j] for j in range(num_suppliers))
  </code></pre>
  
  <p>And I'm trying to show the value of the expression this way:</p>
  
  <pre><code>print('Total cost = ', total_cost)
  </code></pre>
  
  <p>But, instead, what is shown in <strong>print('Total cost = ', total_cost)</strong> is: (I did not put the entire value because the text was too long)</p>
  
  <pre><code>Total cost =  ((((0.0 * &lt;ortools.linear_solver.linear_solver_natural_api.VariableExpr object at 0x7fe188875898&gt;) + (0.0 * &lt;ortools.linear_solver.linear_solver_natural_api.VariableExpr object at 0x7fe188875048&gt;) + (0.0 * &lt;ortools.linear_solver.linear_solver_natural_api.VariableExpr object at 0x7fe188875ac8&gt;) + (0.0 * &lt;ortools.linear_solver.linear_solver_natural_...
  </code></pre>