Adding test (and variables) for sparse matrix building logic

[juaristi22]

Fix #472

[baogorek]

Hi María,

Nice job on this so far. We should be able to get it merged before stand-up. Please see the comments sprinkled throughout. Here is the big one that could reduce the lines of code substantially:

  The TestNationalLevelContributions class (lines 466-656) tests something new and valuable - that national-level targets receive contributions from          
  households across multiple states without geographic bias. This should stay.                                                                                
                                                                                                                                                              
  However, TestStateLevelValues and TestCrossStateRecalculation substantially overlap with the existing test_same_state.py and test_cross_state.py. Could we  
  either:                                                                                                                                                     
  1. Remove the redundant test classes and rely on the existing tests, or                                                                                     
  2. If the new tests add coverage the existing tests miss (e.g., testing more variables), consolidate by extending the existing test files rather than       
  duplicating the approach?                                                                                                                                   
                                                                                                                                                              
  This would reduce the file from ~875 lines to ~300 lines (just configuration, fixtures, helpers, basic structure tests, and TestNationalLevelContributions).

Feel free to get a second opinion on that.

I was puzzled at first by the number of files in the PR but the black "2026 stable style" was released 4 days ago, so your PR gets to be the one to bring in those changes!

[juaristi22]

Sorry I missed the other tests and fixtures. Thank you for pointing me to them! I have removed the redundant tests as you suggested and renamed my file to test_matrix_national_variations.py as the tests became more specific to that. I also took a look at the already existing test_same_state and test_cross_state and made them a bit more flexible so that they automatically consider any new variables we add to conftest.py rather than only checking for snap. Let me know if I missed anything. Otherwise, I think its ready to merge.

[baogorek]

Nice work!

[baogorek]

@juaristi22 the tests were running for 3+ hours so I had to pull this back some. Here's an explanation:

  The core issue is combinatorial explosion in the matrix_data fixture.        

  The SparseMatrixBuilder.build_matrix() method has this computational structure:                                                                             

  For each state (derived from CDs):   
      Create a new Microsimulation (expensive - loads dataset, initializes)    
      Delete all calculated variable arrays                                    
      For each CD in that state:       
          For each target row (one per variable × geographic level):           
              Evaluate constraints                                             
              Calculate variable values for all households                     
                                                                               
  Original config: 4 states × ~5 CDs/state × ~10 SNAP targets = ~200 iterations

  PR config: 8 states × ~3 CDs/state × ~hundreds of targets (30 variables × multiple geo levels) = ~thousands of iterations                                   

  The expensive operations are:        
  1. Creating Microsimulations - each state needs a fresh sim with state_fips set                                                                             
  2. sim.calculate(variable) - called for every target × CD combination        
  3. Constraint evaluation - also calls sim.calculate() for constraint variables                                                                              
                                       
  Going from 1 variable to 30 variables doesn't just multiply time by 30 - it multiplies the number of targets (rows), and each target requires constraint    
  evaluation and variable calculation.                                                                                                                        

  How to avoid this in the future                                                                                                                             
                                       
  1. Keep CI test configs minimal - test with 2-3 representative variables, not the full production list. The full list can live in documentation or a        
  separate "extended tests" workflow.  
  2. Be wary of module-scoped fixtures - the matrix_data fixture builds once per test module, so expanding it affects all tests in that module.               
  3. Consider test markers - use @pytest.mark.slow for expensive tests and skip them in CI by default, running them only on merge to main or nightly.         
  4. Watch the CD count - each additional state requires a new Microsimulation. The original 4 states was chosen deliberately for "manageable size."