def _load_contig_sizes_from_vcf(vcf_fname) -> ChromSizes:
result = dict()
contigs = VariantFile(vcf_fname).header.contigs
for contig_ID, contig_properties in contigs.items():
result[contig_ID] = contig_properties.length
if len(result) == 0:
raise ValueError(
f"{vcf_fname} does not have 'contig' lines giving contig sizes")
return result
