fixes #BITBUCKET-3833 guarding against ratio == 1, adding test, and updating history

HISTORY.rst

@@ -57,6 +57,8 @@ Unreleased Changes
   A DQ score of 1 should represent better data quality whereas the score of 3 is
   worse data quality. A weight score of 1 is more important, whereas that of 3
   is less important.
+* Fixing a case where a zero discount rate and rate of change in the carbon
+  model would cause a divide by zero error.
 
 3.5.0 (2018-08-14)
 ------------------

src/natcap/invest/carbon.py

@@ -169,9 +169,9 @@ def execute(args):
         storage_path_list = []
         for pool_type in ['c_above', 'c_below', 'c_soil', 'c_dead']:
             carbon_pool_by_type = dict([
-                (lucode, float(carbon_pool_table[lucode][pool_type])) 
+                (lucode, float(carbon_pool_table[lucode][pool_type]))
                 for lucode in carbon_pool_table])
-            
+
             lulc_key = 'lulc_%s_path' % scenario_type
             storage_key = '%s_%s' % (pool_type, scenario_type)
             LOGGER.info(
@@ -214,7 +214,7 @@ def execute(args):
             _diff_rasters,
             args=(storage_path_list, file_registry[output_key]),
             target_path_list=[file_registry[output_key]],
-            dependent_task_list=[sum_rasters_task_lookup['cur'], 
+            dependent_task_list=[sum_rasters_task_lookup['cur'],
                 sum_rasters_task_lookup[scenario_type]],
             task_name='diff_rasters_for_%s' % output_key)
         diff_rasters_task_lookup[scenario_type] = diff_rasters_task
@@ -244,10 +244,10 @@ def execute(args):
                 task_name='calculate_%s' % output_key)
             calculate_npv_tasks.append(calculate_npv_task)
             tifs_to_summarize.add(file_registry[output_key])
-    
+
     # Report aggregate results
-    tasks_to_report = (sum_rasters_task_lookup.values() 
-                       + diff_rasters_task_lookup.values() 
+    tasks_to_report = (sum_rasters_task_lookup.values()
+                       + diff_rasters_task_lookup.values()
                        + calculate_npv_tasks)
     generate_report_task = graph.add_task(
         _generate_report,
@@ -363,8 +363,9 @@ def _calculate_valuation_constant(
                (1 + float(rate_change) / 100.0)))
     valuation_constant = (
         float(price_per_metric_ton_of_c) /
-        (float(lulc_fut_year) - float(lulc_cur_year)) *
-        (1.0 - ratio ** (n_years + 1)) / (1.0 - ratio))
+        (float(lulc_fut_year) - float(lulc_cur_year)))
+    if ratio != 1.0:
+        valuation_constant *= (1.0 - ratio ** (n_years + 1)) / (1.0 - ratio)
     return valuation_constant
 
 

tests/test_carbon.py

@@ -121,6 +121,41 @@ class CarbonTests(unittest.TestCase):
         assert_raster_equal_value(
             os.path.join(args['workspace_dir'], 'npv_redd.tif'), -0.4602106)
 
+    def test_carbon_zero_rates(self):
+        """Carbon: test with 0 discount and rate change."""
+        from natcap.invest import carbon
+
+        args = {
+            u'workspace_dir': self.workspace_dir,
+            u'do_valuation': True,
+            u'price_per_metric_ton_of_c': 43.0,
+            u'rate_change': 0.0,
+            u'lulc_cur_year': 2016,
+            u'lulc_fut_year': 2030,
+            u'discount_rate': 0.0,
+            u'n_workers': -1,
+        }
+
+        # Create LULC rasters and pools csv in workspace and add them to args.
+        lulc_names = ['lulc_cur_path', 'lulc_fut_path', 'lulc_redd_path']
+        for fill_val, lulc_name in enumerate(lulc_names, 1):
+            args[lulc_name] = os.path.join(args['workspace_dir'],
+                                           lulc_name + '.tif')
+            make_simple_raster(args[lulc_name], fill_val)
+
+        args['carbon_pools_path'] = os.path.join(args['workspace_dir'],
+                                                 'pools.csv')
+        make_pools_csv(args['carbon_pools_path'])
+
+        carbon.execute(args)
+
+        # Add assertions for npv for future and REDD scenarios.
+        # The npv was calculated based on _calculate_npv in carbon.py.
+        assert_raster_equal_value(
+            os.path.join(args['workspace_dir'], 'npv_fut.tif'), -0.0178143)
+        assert_raster_equal_value(
+            os.path.join(args['workspace_dir'], 'npv_redd.tif'), -0.0239571)
+
     def test_carbon_future(self):
         """Carbon: regression testing future scenario."""
         from natcap.invest import carbon