title: How to code a constraint in Gurobi python
body: <p>I am so new in python and Gurobi!
  recently I started to code a problem and I need to code this constraints:</p>
  
  <pre><code>for each i and j (while i is not equal to j):
  (y[i][j]) * (D[i] - d[i][j])) &lt;= T
  </code></pre>
  
  <p>Also I tried to start with not considering the equal part and wrote:</p>
  
  <pre><code>for i in people:
       for j in people:
            m.addConstr((carpools[i][j])*(distance[i]-distBtw[i][j])) &lt;= 400
  </code></pre>
  
  <p>but I received this error:</p>
  
  <pre><code>TypeError: unsupported operand type(s) for *: 'int' and 'NoneType'
  </code></pre>