Failure to patch accurately

test.py

import unittest
from mock import patch

class Class:
    def __init__(self):
        self.foo = 'bar'

class TestClass(unittest.TestCase):
    @patch('__main__.Class')
    def test_class(self, MockClass):
        mock_class = MockClass.return_value
        mock_class.foo = 'THIS IS NOT BAR ANYMORE, SNACKY.'
        self.assertTrue(mock_class.foo != 'bar')