Issue a warning if the number of alt alleles exceeds the maximum specified

Author: tomwhite

sgkit/io/utils.py

@@ -104,6 +104,11 @@ def zarrs_to_dataset(
             ds[variable_name] = ds[variable_name].astype(f"S{max_length}")
             del ds.attrs[attr]
 
+    if "max_alt_alleles_seen" in datasets[0].attrs:
+        ds.attrs["max_alt_alleles_seen"] = max(
+            ds.attrs["max_alt_alleles_seen"] for ds in datasets
+        )
+
     return ds
 
 

sgkit/io/vcf/__init__.py

@@ -3,9 +3,10 @@
 try:
     from ..utils import zarrs_to_dataset
     from .vcf_partition import partition_into_regions
-    from .vcf_reader import vcf_to_zarr, vcf_to_zarrs
+    from .vcf_reader import MaxAltAllelesExceededWarning, vcf_to_zarr, vcf_to_zarrs
 
     __all__ = [
+        "MaxAltAllelesExceededWarning",
         "partition_into_regions",
         "vcf_to_zarr",
         "vcf_to_zarrs",

sgkit/io/vcf/vcf_reader.py

@@ -1,5 +1,6 @@
 import functools
 import itertools
+import warnings
 from contextlib import contextmanager
 from dataclasses import dataclass
 from pathlib import Path
@@ -21,6 +22,7 @@
 import xarray as xr
 from cyvcf2 import VCF, Variant
 
+from sgkit.io.dataset import load_dataset
 from sgkit.io.utils import zarrs_to_dataset
 from sgkit.io.vcf import partition_into_regions
 from sgkit.io.vcf.utils import build_url, chunks, temporary_directory, url_filename
@@ -34,6 +36,12 @@
 )
 
 
+class MaxAltAllelesExceededWarning(UserWarning):
+    """Warning when the number of alt alleles exceeds the maximum specified."""
+
+    pass
+
+
 @contextmanager
 def open_vcf(path: PathType) -> Iterator[VCF]:
     """A context manager for opening a VCF file."""
@@ -256,6 +264,7 @@ def vcf_to_zarr_sequential(
         # Remember max lengths of variable-length strings
         max_variant_id_length = 0
         max_variant_allele_length = 0
+        max_alt_alleles_seen = 0
 
         # Iterate through variants in batches of chunk_length
 
@@ -303,6 +312,7 @@ def vcf_to_zarr_sequential(
                 variant_position[i] = variant.POS
 
                 alleles = [variant.REF] + variant.ALT
+                max_alt_alleles_seen = max(max_alt_alleles_seen, len(variant.ALT))
                 if len(alleles) > n_allele:
                     alleles = alleles[:n_allele]
                 elif len(alleles) < n_allele:
@@ -359,6 +369,7 @@ def vcf_to_zarr_sequential(
             if add_str_max_length_attrs:
                 ds.attrs["max_length_variant_id"] = max_variant_id_length
                 ds.attrs["max_length_variant_allele"] = max_variant_allele_length
+            ds.attrs["max_alt_alleles_seen"] = max_alt_alleles_seen
 
             if first_variants_chunk:
                 # Enforce uniform chunks in the variants dimension
@@ -705,6 +716,15 @@ def vcf_to_zarr(
         field_defs=field_defs,
     )
 
+    # Issue a warning if max_alt_alleles caused data to be dropped
+    ds = load_dataset(output)
+    max_alt_alleles_seen = ds.attrs["max_alt_alleles_seen"]
+    if max_alt_alleles_seen > max_alt_alleles:
+        warnings.warn(
+            f"Some alternate alleles were dropped, since actual max value {max_alt_alleles_seen} exceeded max_alt_alleles setting of {max_alt_alleles}.",
+            MaxAltAllelesExceededWarning,
+        )
+
 
 def count_variants(path: PathType, region: Optional[str] = None) -> int:
     """Count the number of variants in a VCF file."""

sgkit/tests/io/vcf/test_vcf_reader.py

@@ -6,7 +6,11 @@
 from numpy.testing import assert_allclose, assert_array_equal
 
 from sgkit import load_dataset
-from sgkit.io.vcf import partition_into_regions, vcf_to_zarr
+from sgkit.io.vcf import (
+    MaxAltAllelesExceededWarning,
+    partition_into_regions,
+    vcf_to_zarr,
+)
 
 from .utils import path_for_test
 
@@ -96,30 +100,35 @@ def test_vcf_to_zarr__max_alt_alleles(shared_datadir, is_path, tmp_path):
     path = path_for_test(shared_datadir, "sample.vcf.gz", is_path)
     output = tmp_path.joinpath("vcf.zarr").as_posix()
 
-    vcf_to_zarr(path, output, chunk_length=5, chunk_width=2, max_alt_alleles=1)
-    ds = xr.open_zarr(output)  # type: ignore[no-untyped-call]
+    with pytest.warns(MaxAltAllelesExceededWarning):
+        vcf_to_zarr(path, output, chunk_length=5, chunk_width=2, max_alt_alleles=1)
+        ds = xr.open_zarr(output)  # type: ignore[no-untyped-call]
 
-    # extra alt alleles are silently dropped
-    assert_array_equal(
-        ds["variant_allele"],
-        [
-            ["A", "C"],
-            ["A", "G"],
-            ["G", "A"],
-            ["T", "A"],
-            ["A", "G"],
-            ["T", ""],
-            ["G", "GA"],
-            ["T", ""],
-            ["AC", "A"],
-        ],
-    )
+        # extra alt alleles are dropped
+        assert_array_equal(
+            ds["variant_allele"],
+            [
+                ["A", "C"],
+                ["A", "G"],
+                ["G", "A"],
+                ["T", "A"],
+                ["A", "G"],
+                ["T", ""],
+                ["G", "GA"],
+                ["T", ""],
+                ["AC", "A"],
+            ],
+        )
+
+        # the maximum number of alt alleles actually seen is stored as an attribute
+        assert ds.attrs["max_alt_alleles_seen"] == 3
 
 
 @pytest.mark.parametrize(
     "is_path",
     [True, False],
 )
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__large_vcf(shared_datadir, is_path, tmp_path):
     path = path_for_test(shared_datadir, "CEUTrio.20.21.gatk3.4.g.vcf.bgz", is_path)
     output = tmp_path.joinpath("vcf.zarr").as_posix()
@@ -157,6 +166,7 @@ def test_vcf_to_zarr__plain_vcf_with_no_index(shared_datadir, tmp_path):
     "is_path",
     [True, False],
 )
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__mutable_mapping(shared_datadir, is_path):
     path = path_for_test(shared_datadir, "CEUTrio.20.21.gatk3.4.g.vcf.bgz", is_path)
     output: MutableMapping[str, bytes] = {}
@@ -182,6 +192,7 @@ def test_vcf_to_zarr__mutable_mapping(shared_datadir, is_path):
     "is_path",
     [True, False],
 )
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__parallel(shared_datadir, is_path, tmp_path):
     path = path_for_test(shared_datadir, "CEUTrio.20.21.gatk3.4.g.vcf.bgz", is_path)
     output = tmp_path.joinpath("vcf_concat.zarr").as_posix()
@@ -208,6 +219,7 @@ def test_vcf_to_zarr__parallel(shared_datadir, is_path, tmp_path):
     "is_path",
     [False],
 )
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__parallel_temp_chunk_length(shared_datadir, is_path, tmp_path):
     path = path_for_test(shared_datadir, "CEUTrio.20.21.gatk3.4.g.vcf.bgz", is_path)
     output = tmp_path.joinpath("vcf_concat.zarr").as_posix()
@@ -296,6 +308,7 @@ def test_vcf_to_zarr__parallel_partitioned_by_size(shared_datadir, is_path, tmp_
     "is_path",
     [True, False],
 )
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__multiple(shared_datadir, is_path, tmp_path):
     paths = [
         path_for_test(shared_datadir, "CEUTrio.20.gatk3.4.g.vcf.bgz", is_path),
@@ -323,6 +336,7 @@ def test_vcf_to_zarr__multiple(shared_datadir, is_path, tmp_path):
     "is_path",
     [True, False],
 )
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__multiple_partitioned(shared_datadir, is_path, tmp_path):
     paths = [
         path_for_test(shared_datadir, "CEUTrio.20.gatk3.4.g.vcf.bgz", is_path),
@@ -352,6 +366,7 @@ def test_vcf_to_zarr__multiple_partitioned(shared_datadir, is_path, tmp_path):
     "is_path",
     [True, False],
 )
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__multiple_partitioned_by_size(shared_datadir, is_path, tmp_path):
     paths = [
         path_for_test(shared_datadir, "CEUTrio.20.gatk3.4.g.vcf.bgz", is_path),
@@ -398,6 +413,31 @@ def test_vcf_to_zarr__mutiple_partitioned_invalid_regions(
         vcf_to_zarr(paths, output, regions=regions, chunk_length=5_000)
 
 
+@pytest.mark.parametrize(
+    "is_path",
+    [True, False],
+)
+def test_vcf_to_zarr__multiple_max_alt_alleles(shared_datadir, is_path, tmp_path):
+    paths = [
+        path_for_test(shared_datadir, "CEUTrio.20.gatk3.4.g.vcf.bgz", is_path),
+        path_for_test(shared_datadir, "CEUTrio.21.gatk3.4.g.vcf.bgz", is_path),
+    ]
+    output = tmp_path.joinpath("vcf_concat.zarr").as_posix()
+
+    with pytest.warns(MaxAltAllelesExceededWarning):
+        vcf_to_zarr(
+            paths,
+            output,
+            target_part_size="40KB",
+            chunk_length=5_000,
+            max_alt_alleles=1,
+        )
+        ds = xr.open_zarr(output)  # type: ignore[no-untyped-call]
+
+        # the maximum number of alt alleles actually seen is stored as an attribute
+        assert ds.attrs["max_alt_alleles_seen"] == 7
+
+
 @pytest.mark.parametrize(
     "ploidy,mixed_ploidy,truncate_calls,regions",
     [
@@ -560,6 +600,7 @@ def test_vcf_to_zarr__fields(shared_datadir, tmp_path):
     assert ds["call_DP"].attrs["comment"] == "Read Depth"
 
 
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__parallel_with_fields(shared_datadir, tmp_path):
     path = path_for_test(shared_datadir, "CEUTrio.20.21.gatk3.4.g.vcf.bgz")
     output = tmp_path.joinpath("vcf.zarr").as_posix()
@@ -616,6 +657,7 @@ def test_vcf_to_zarr__field_defs(shared_datadir, tmp_path):
     assert "comment" not in ds["variant_DP"].attrs
 
 
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__field_number_A(shared_datadir, tmp_path):
     path = path_for_test(shared_datadir, "sample.vcf.gz")
     output = tmp_path.joinpath("vcf.zarr").as_posix()
@@ -649,6 +691,7 @@ def test_vcf_to_zarr__field_number_A(shared_datadir, tmp_path):
     )
 
 
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_vcf_to_zarr__field_number_R(shared_datadir, tmp_path):
     path = path_for_test(shared_datadir, "CEUTrio.21.gatk3.4.g.vcf.bgz")
     output = tmp_path.joinpath("vcf.zarr").as_posix()

sgkit/tests/io/vcf/test_vcf_roundtrip.py

@@ -79,6 +79,7 @@ def test_default_fields(shared_datadir, tmpdir):
     sg_vcfzarr_path = create_sg_vcfzarr(shared_datadir, tmpdir)
     sg_ds = sg.load_dataset(str(sg_vcfzarr_path))
     sg_ds = sg_ds.drop_vars("call_genotype_phased")  # not included in scikit-allel
+    del sg_ds.attrs["max_alt_alleles_seen"]  # not saved by scikit-allel
 
     assert_identical(allel_ds, sg_ds)
 
@@ -107,6 +108,7 @@ def test_DP_field(shared_datadir, tmpdir):
     )
     sg_ds = sg.load_dataset(str(sg_vcfzarr_path))
     sg_ds = sg_ds.drop_vars("call_genotype_phased")  # not included in scikit-allel
+    del sg_ds.attrs["max_alt_alleles_seen"]  # not saved by scikit-allel
 
     assert_identical(allel_ds, sg_ds)
 
@@ -120,6 +122,7 @@ def test_DP_field(shared_datadir, tmpdir):
         ("CEUTrio.20.21.gatk3.4.g.vcf.bgz", ["calldata/PL"], ["FORMAT/PL"]),
     ],
 )
+@pytest.mark.filterwarnings("ignore::sgkit.io.vcf.MaxAltAllelesExceededWarning")
 def test_all_fields(
     shared_datadir, tmpdir, vcf_file, allel_exclude_fields, sgkit_exclude_fields
 ):
@@ -159,6 +162,7 @@ def test_all_fields(
     )
     sg_ds = sg.load_dataset(str(sg_vcfzarr_path))
     sg_ds = sg_ds.drop_vars("call_genotype_phased")  # not included in scikit-allel
+    del sg_ds.attrs["max_alt_alleles_seen"]  # not saved by scikit-allel
 
     # scikit-allel only records contigs for which there are actual variants,
     # whereas sgkit records contigs from the header