python -m unittest发现不工作

Question

我正在使用优秀的Tox工具来测试我的代码。这是我的tox.ini：

[tox]
envlist = py26,py27,py31,py32,py33

[testenv]
deps = ParamUnittest
commands = python -m unittest discover tests
downloadcache = build

[testenv:py26]
deps = https://github.com/rik0/ParamUnittest/tarball/master
       unittest2
       importlib
commands = python -m unittest2.__main__ discover tests

[testenv:py31]
deps = https://github.com/rik0/ParamUnittest/tarball/master
       unittest2py3k
commands = python -m unittest2 discover tests

运行时，除3.1之外的所有版本都执行所有测试。 Tox表示：

py31 sdist-reinst: /home/miki/exp/radon/.tox/dist/radon-0.1.zip
py31 runtests: commands[0]

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

OK

为什么不执行测试？系统布局如下：

tests/
    test_internals.py
    test_complexity.py
    ...
radon/
...
setup.py
tox.ini