title: Adding two `csc` sparse matrices of different shapes in python
body: <p>So I have two <code>csc</code> matrices of different shapes that I need to add together. The matrices look like this:</p>
  
  <pre><code>current_flows = (7005, 1001)    50.0
  (8259, 1001)    65.0
  (14007, 1001)   45.0
  (9971, 1002)    80.0
  : :
  (69003, 211148) 0.0
  
  result_flows = (7005, 1001) 40
  (14007, 1001)   20
  (9971, 1002)    35
  : :
  (71136, 71137)  90
  
  final_flows = current_flows + result_flows
  </code></pre>
  
  <p>As illustrated by some of the row and column ID's shown: <code>(7005, 1001), (14007, 1001), (9971, 1002)</code> the matrices do have elements in common.  Based on their final row and column ID's though they are of different shape.  </p>
  
  <p>I would like to add the two matrices together, while preserving the shape of the larger matrix (<code>current_flows</code>) and keeping the values of <code>current_flows</code> the same where <code>result_flows</code> does not have a row and column ID that matches <code>current_flows</code>. So, <code>final_flows</code> will have row and column indexes extending to: <code>(69003, 211148)</code> even though <code>result_flows</code> only extends to <code>(71136, 71137)</code></p>
  
  <p>Thus, I would like my output to be:</p>
  
  <pre><code>final_flows =  (7005, 1001) 90.0
  (8259, 1001)    65.0
  (14007, 1001)   65.0
  (9971, 1002)    115.0
  : :
  (71136, 71137)  90
  (69003, 211148) 0.0
  </code></pre>
  
  <p>Let me know if you'd like any further clarification, thanks!</p>