Mock在Python 2.7的unittest中不起作用

我的项目结构如下:

py_test
   |
   +---my_module
   |    |  
   |    +--- __init__.py (empty)
   |    |
   |    +--- futil.py
   |
   +---test
        |
        +--- __init__.py (empty)
        |
        +--- futil_test.py

在 无用的.py 我有以下资料:

from os import path


def check_exists(file_path):
    return path.exists(file_path)

在 futil_test.py测试 我试图实现这样的单元测试:

import mock
import unittest

from my_module.futil import check_exists


class TestExists(unittest.TestCase):

    @mock.patch('my_module.futil.os.path')        # <---leads to error, as well as my_module.os.path
    def test_exists(self, mock_path):
        mock_path.exists.return_value = True
        self.assertTrue(check_exists('ba'))


if __name__ == '__main__':
    unittest.main()

当我尝试启动单元测试时,它失败并出现错误:

Error
Traceback (most recent call last):
  File "/usr/lib64/python2.7/unittest/case.py", line 367, in run
    testMethod()
  File "/home/antonio/devel/py/py_test/venv/lib/python2.7/site-packages/mock/mock.py", line 1322, in patched
    arg = patching.__enter__()
  File "/home/antonio/devel/py/py_test/venv/lib/python2.7/site-packages/mock/mock.py", line 1378, in __enter__
    self.target = self.getter()
  File "/home/antonio/devel/py/py_test/venv/lib/python2.7/site-packages/mock/mock.py", line 1548, in <lambda>
    getter = lambda: _importer(target)
  File "/home/antonio/devel/py/py_test/venv/lib/python2.7/site-packages/mock/mock.py", line 1235, in _importer
    thing = _dot_lookup(thing, comp, import_path)
  File "/home/antonio/devel/py/py_test/venv/lib/python2.7/site-packages/mock/mock.py", line 1224, in _dot_lookup
    __import__(import_path)
ImportError: No module named os

在这个例子中 here 类似的建设似乎也在起作用。我做错什么了?