import builtins
import operator

from itertools import ifilter
from random import shuffle


class Backend(object):
    def __init__(self, priority):
        super(Backend, self).__init__()
        self.priority = priority


def max(*args, **kwargs):
    """
    Add support for 'default' kwarg.

    >>> max([], default='res')
    'res'

    >>> max(default='res')
    Traceback (most recent call last):
    ...
    TypeError: ...

    >>> max('a', 'b', default='other')
    'b'
    """
    missing = object()
    default = kwargs.pop('default', missing)
    try:
        mmax = builtins.max(*args, **kwargs)
        return mmax
    except ValueError as exc:
        if 'empty sequence' in str(exc) and default is not missing:
            return default
        raise


by_priority = operator.attrgetter('priority')

backends = [Backend(5), Backend(1), Backend(0.5), Backend(0.6)]

while True:
    shuffle(backends)
    filtered_backends = ifilter(None, backends)
    max_object = max(filtered_backends, key=by_priority)
    print(max_object.priority)
    if max_object.priority != 5:
        exit(1)