Retrieve interval to run analysis in. Handles no targets, BED and regions

    region can be a single region or list of multiple regions for multicore calling.

def _get_interval(variant_regions, region, out_file, items):
    """Retrieve interval to run analysis in. Handles no targets, BED and regions

    region can be a single region or list of multiple regions for multicore calling.
    """
    target = shared.subset_variant_regions(variant_regions, region, out_file, items)
    if target:
        if isinstance(target, six.string_types) and os.path.isfile(target):
            return "--interval %s" % target
        else:
            return "--interval %s" % bamprep.region_to_gatk(target)
    else:
        return ""