Class decorator that fills in missing ordering methods

def total_ordering(cls):
    """Class decorator that fills in missing ordering methods"""
    convert = {  # :off
        '__lt__': [('__gt__', _gt_from_lt),
                   ('__le__', _le_from_lt),
                   ('__ge__', _ge_from_lt)],
        '__le__': [('__ge__', _ge_from_le),
                   ('__lt__', _lt_from_le),
                   ('__gt__', _gt_from_le)],
        '__gt__': [('__lt__', _lt_from_gt),
                   ('__ge__', _ge_from_gt),
                   ('__le__', _le_from_gt)],
        '__ge__': [('__le__', _le_from_ge),
                   ('__gt__', _gt_from_ge),
                   ('__lt__', _lt_from_ge)]
    }  # :on
    # Find user-defined comparisons (not those inherited from object).
    roots = [  # :off
        op
        for op in convert
        if getattr(cls, op, None) is not getattr(object, op, None)
    ]  # :on
    if not roots:
        raise ValueError('must define at least one ordering operation: < > <= >=')
    root = max(roots)  # prefer __lt__ to __le__ to __gt__ to __ge__
    for opname, opfunc in convert[root]:
        if opname not in roots:
            opfunc.__name__ = opname
            setattr(cls, opname, opfunc)
    return cls