fix: dedent empty-data check in cnv_discordant_read_calls (#777)

The if len(ly) == 0 check was incorrectly indented inside the inner
or s in prepared_sample_sets loop, making it dead code. This caused
an IndexError (from _simple_xarray_concat on an empty list) instead
of the intended ValueError when no data was found for a contig.

- Dedent the check to sit outside the inner loop (matching cnv_hmm)
- Improve error message to include the contig name
- Update test to verify the specific error message via match=

Author: AswaniSahoo

malariagen_data/anoph/cnv_data.py

@@ -642,9 +642,13 @@ def cnv_discordant_read_calls(
 
                 ly.append(y)
 
-                if len(ly) == 0:
-                    # Bail out, no data for given sample sets and analysis.
-                    raise ValueError("No data found for requested sample sets.")
+            if len(ly) == 0:
+                # Bail out, no data for given sample sets and contig.
+                raise ValueError(
+                    f"No CNV discordant read calls data found for contig {c!r} "
+                    f"in the requested sample sets. This could be because the "
+                    f"sample sets do not have discordant read calls data available."
+                )
 
             x = _simple_xarray_concat(ly, dim=DIM_SAMPLE)
             lx.append(x)

tests/anoph/test_cnv_data.py

@@ -626,7 +626,10 @@ def test_cnv_discordant_read_calls(fixture, api: AnophelesCnvData):
             assert isinstance(d2, xr.DataArray)
 
     # Check with a contig that should not exist
-    with pytest.raises(ValueError):
+    with pytest.raises(
+        ValueError,
+        match="No CNV discordant read calls data found|no CNVs available for contig",
+    ):
         api.cnv_discordant_read_calls(
             contig="foobar", sample_sets=random.choice(all_sample_sets)
         )