Reset password hashers cache on settings_changed.

import hashlib

from django.dispatch import receiver

from django.conf import settings

from django.test.signals import setting_changed

from django.utils import importlib

from django.utils.datastructures import SortedDict

from django.utils.encoding import smart_str

HASHERS = None  # lazily loaded from PASSWORD_HASHERS

PREFERRED_HASHER = None  # defaults to first item in PASSWORD_HASHERS

@receiver(setting_changed)

def reset_hashers(**kwargs):

    if kwargs['setting'] == 'PASSWORD_HASHERS':

        global HASHERS, PREFERRED_HASHER

        HASHERS = None

        PREFERRED_HASHER = None

def is_password_usable(encoded):

    return (encoded is not None and encoded != UNUSABLE_PASSWORD)