The correct format for operator_range is <start>-<end>, or <point>, <start>-<end>

Raise code

_operator_range(chars_range):
    """Generates the characters from chars_range inclusive."""
    if chars_range == 'None' or chars_range is None:
        return None

    if all(item not in chars_range for item in [',', '-']):
        raise ValueError("The correct format for operator_range is "
                         "<start>-<end>, or <point>, <start>-<end>")

    ops_start_chars_set = set()
    ranges = chars_range.split(',')
    for item in ranges:
        if len(item) == 1:
            ops_start_chars_set.add(item.lower())