Catch case where new Hazard fraction is unintentionally zero

CHANGELOG.md

@@ -18,6 +18,8 @@ Code freeze date: YYYY-MM-DD
 
 ### Fixed
 
+- Avoid an issue where a Hazard subselection would have a fraction matrix with only zeros as entries by throwing an error [#866](https://github.com/CLIMADA-project/climada_python/pull/866)
+
 ### Deprecated
 
 ### Removed

climada/hazard/base.py

@@ -1457,6 +1457,18 @@ def select(self, event_names=None, event_id=None, date=None, orig=None,
                                    dt.datetime.fromordinal(haz.date.min()).year) + 1
             haz.frequency = haz.frequency * year_span_old / year_span_new
 
+        # Check if new fraction is zero everywhere
+        if self.fraction.nnz != 0 and haz.fraction.nnz == 0:
+            raise RuntimeError(
+                "Your selection created a Hazard object where the fraction matrix is "
+                "zero everywhere. This hazard will have zero impact everywhere. "
+                "We are catching this condition because of an implementation detail: "
+                "A fraction matrix without nonzero-valued entries will be completely "
+                "ignored. This is surely not what you intended. If you really want to, "
+                "you can circumvent this error by setting your original fraction "
+                "matrix to zero everywhere, but there probably is no point in doing so."
+            )
+
         haz.sanitize_event_ids()
         return haz
 

climada/hazard/test/test_base.py

@@ -601,6 +601,19 @@ def test_select_tight_pass(self):
         self.assertIsInstance(sel_haz.intensity, sparse.csr_matrix)
         self.assertIsInstance(sel_haz.fraction, sparse.csr_matrix)
 
+    def test_select_new_fraction_zero(self):
+        """Check if a new fraction of only zeros is handled correctly"""
+        hazard = dummy_hazard()
+        hazard.centroids.region_id = np.array([1, 1, 2])
+
+        # Select a part of the hazard where fraction is zero only
+        with self.assertRaisesRegex(
+            RuntimeError,
+            "Your selection created a Hazard object where the fraction matrix is zero "
+            "everywhere"
+        ):
+            hazard.select(event_id=[3, 4], reg_id=[2])
+
 
 class TestAppend(unittest.TestCase):
     """Test append method."""