在pytest 2.0以上的版本里，我们也可以通过python -m pytest ...来调用。这实际上和pytest 。。。几乎一摸一样。
只是用python的时候，把当面目录也加入到sys.path中。
pytest的返回值如下：
Exit code 0: All tests were collected and passed successfully
Exit code 1: Tests were collected and run but some of the tests failed
Exit code 2: Test execution was interrupted by the user
Exit code 3: Internal error happened while executing tests
Exit code 4: pytest command line usage error
Exit code 5: No tests were collected
pytest的一些常用的命令你：
pytest --version 显示pytest import的版本，以及已经注册的插件。
C:\fitme\httpie\tests>pytest --version
This is pytest version 3.1.3, imported from c:\python35\lib\site-packages\pytest.py
setuptools registered plugins:
pytest-httpbin-0.2.3 at c:\python35\lib\site-packages\pytest_httpbin\plugin.py
pytest 碰到第几个错误停止：
pytest -x # stop after first failure
pytest --maxfail=2 # stop after two failures
指定路径啊，文件啊，文件里的函数，类等进行测试。
pytest test_mod.py # run tests in module
pytest somepath # run all tests below somepath
pytest -k stringexpr # only run tests with names that match the
# "string expression", e.g. "MyClass and not method"
# will select TestMyClass.test_something
# but not TestMyClass.test_method_simple
pytest test_mod.py::test_func # only run tests that match the "node ID",
# e.g. "test_mod.py::test_func" will select
# only test_func in test_mod.py
pytest test_mod.py::TestClass::test_method # run a single method in
# a single class