def only_call(variant):
  """Ensures the Variant has exactly one VariantCall, and returns it.

  Args:
    variant: nucleus.genomics.v1.Variant. The variant of interest.

  Returns:
    The single nucleus.genomics.v1.VariantCall in the variant.

  Raises:
    ValueError: Not exactly one VariantCall is in the variant.
  """
  if len(variant.calls) != 1:
    raise ValueError('Expected exactly one VariantCall in {}'.format(variant))
  return variant.calls[0]