x.py

class DomainMap:

    _map = None

    @classmethod
    def _get(cls):
        raise NotImplementedError('heavy work done here (someday)')

    @classmethod
    def get(cls):
        if cls._map is None:
            cls._map = cls._get()

        return cls._map

    @classmethod
    def reset(cls):
        cls._map = None


#####
from unittest import mock
import unittest

class TestDomainMap(unittest.TestCase):

    def test_singleton_and_reset(self):

        with mock.patch('x.DomainMap._get', mock.Mock(return_value={'foo': 'boo'})):
            x = DomainMap.get()

        self.assertEquals(x, {'foo': 'boo'})

        with mock.patch('x.DomainMap._get', mock.Mock(return_value={'coo': 'doo'})):
            x = DomainMap.get()

        self.assertEquals(x, {'foo': 'boo'})  # old value, cached!

        DomainMap.reset()

        with mock.patch('x.DomainMap._get', mock.Mock(return_value={'coo': 'doo'})):
            x = DomainMap.get()

        self.assertEquals(x, {'coo': 'doo'})  # new value, reset worked!