fix: sum_of_base_qualities() returns zero when a record has no base…

… qualities (#212)

Closes #210

fgpyo/sam/__init__.py

@@ -158,6 +158,7 @@
 import enum
 import io
 import sys
+from array import array
 from collections.abc import Collection
 from itertools import chain
 from pathlib import Path
@@ -178,6 +179,7 @@
 from pysam import AlignedSegment
 from pysam import AlignmentFile as SamFile
 from pysam import AlignmentHeader as SamHeader
+from pysam import qualitystring_to_array
 from typing_extensions import deprecated
 
 import fgpyo.io
@@ -189,12 +191,18 @@
 NO_REF_INDEX: int = -1
 """The reference index to use to indicate no reference in SAM/BAM."""
 
-NO_REF_NAME: str = "*"
+STRING_PLACEHOLDER: str = "*"
+"""The value to use when a string field's information is unavailable."""
+
+NO_REF_NAME: str = STRING_PLACEHOLDER
 """The reference name to use to indicate no reference in SAM/BAM."""
 
 NO_REF_POS: int = -1
 """The reference position to use to indicate no position in SAM/BAM."""
 
+NO_QUERY_QUALITIES: array = qualitystring_to_array(STRING_PLACEHOLDER)
+"""The quality array corresponding to an unavailable query quality string ("*")."""
+
 _IOClasses = (io.TextIOBase, io.BufferedIOBase, io.RawIOBase, io.IOBase)
 """The classes that should be treated as file-like classes"""
 
@@ -849,16 +857,24 @@ def from_read(cls, read: pysam.AlignedSegment) -> List["SupplementaryAlignment"]
 def sum_of_base_qualities(rec: AlignedSegment, min_quality_score: int = 15) -> int:
     """Calculate the sum of base qualities score for an alignment record.
 
-    This function is useful for calculating the "mate score" as implemented in samtools fixmate.
+    This function is useful for calculating the "mate score" as implemented in `samtools fixmate`.
+    Consistently with `samtools fixmate`, this function returns 0 if the record has no base
+    qualities.
 
     Args:
         rec: The alignment record to calculate the sum of base qualities from.
         min_quality_score: The minimum base quality score to use for summation.
 
+    Returns:
+        The sum of base qualities on the input record. 0 if the record has no base qualities.
+
     See:
         [`calc_sum_of_base_qualities()`](https://github.com/samtools/samtools/blob/4f3a7397a1f841020074c0048c503a01a52d5fa2/bam_mate.c#L227-L238)
         [`MD_MIN_QUALITY`](https://github.com/samtools/samtools/blob/4f3a7397a1f841020074c0048c503a01a52d5fa2/bam_mate.c#L42)
     """
+    if rec.query_qualities is None or rec.query_qualities == NO_QUERY_QUALITIES:
+        return 0
+
     score: int = sum(qual for qual in rec.query_qualities if qual >= min_quality_score)
     return score
 

tests/fgpyo/sam/test_sam.py

@@ -10,6 +10,8 @@
 
 import pysam
 import pytest
+from pysam import AlignedSegment
+from pysam import AlignmentFile
 from pysam import AlignmentHeader
 
 import fgpyo.sam as sam
@@ -616,6 +618,32 @@ def test_sum_of_base_qualities_some_below_minimum() -> None:
     assert sum_of_base_qualities(single, min_quality_score=4) == 9
 
 
+def test_sum_of_base_qualities_unmapped(tmp_path: Path) -> None:
+    builder = SamBuilder(r1_len=5, r2_len=5)
+    single: AlignedSegment = builder.add_single()
+
+    # NB: assigning to `query_qualities` does not affect the cached object returned by the property,
+    # but it does change the underlying attribute used when constructing the string representation
+    single.query_qualities = None
+    record_str = single.to_string()
+
+    bam_path: Path = tmp_path / "no_qualities.bam"
+
+    with bam_path.open("w") as bam_file:
+        bam_file.write(str(builder.header))
+        bam_file.write(f"{record_str}\n")
+
+    with AlignmentFile(str(bam_path)) as bam:
+        record = next(bam)
+
+    # NB: writing to the temp file above is necessary to construct an `AlignedSegment` with no base
+    # qualities, as `SamBuilder` does not currently permit the construction of such records.
+    # TODO simplify this after `SamBuilder` is updated to support this.
+    # https://github.com/fulcrumgenomics/fgpyo/issues/211
+    assert record.query_qualities is None
+    assert sum_of_base_qualities(record) == 0
+
+
 def test_calc_edit_info_no_edits() -> None:
     chrom = "ACGCTAGACTGCTAGCAGCATCTCATAGCACTTCGCGCTATAGCGATATAAATATCGCGATCTAGCG"
     builder = SamBuilder(r1_len=30)