当测试文件位于不同的模块中时,Python无法找到测试

我有一个目录结构如下

horizontalupgrade
  common/
    __init__.py
    upgradestate.py
  tests/
    common/
      __init__.py
      testupgradestate.py

的内容 testupgradestate.py

import unittest
from upgradestate import UpgradeState

class UpgradeStateTest(unittest.TestCase):

    def setUp(self):
        print "Setup Called"

    def test_copy(self):
        u = UpgradeState("")
        print "test_copy Called"

if __name__ == '__main__':
    suite = unittest.TestLoader().loadTestsFromTestCase(UpgradeStateTest)
    runner = unittest.TextTestRunner()
    runner.run(suite)

但是在尝试执行测试时,python无法找到测试

(venv) dmanna-a01:horizontalupgrade dmanna$ python -m unittest discover  -v

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK

但如果我把我的目录结构如下

horizontalupgrade
  common/
    __init__.py
    upgradestate.py
    testupgradestate.py

那么测试运行正常

(venv) dmanna-a01:horizontalupgrade dmanna$ python -m unittest discover  -v
test_copy (common.testupgradestate.UpgradeStateTest) ... Setup Called
test_copy Called
ok

----------------------------------------------------------------------
Ran 1 tests in 0.000s

OK

• Python-2.7版