python 'try/except' in unittest?

起初，我在一个 test function 中定义的测试流程如下：

def setUp(self):
    try:
        self.head = Tree('root', data='head.data', extra='head.extra')
        self.failUnlessEqual(self.head._Tree__node_value, 'root')
        self.failUnlessEqual(self.head.data, 'head.data')  #(1)
        self.failUnlessEqual(self.head.extra, 'head.extra')
    except:
        seilf.fail(_("Unexpected exception catched"))

然后我运行这个单元测试：

sh$ python tree_ut.py
EEEE
======================================================================
ERROR: test__call__ (__main__.TestTree)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tree_ut.py", line 29, in setUp
    self.fail(_("Unexpected exception catched"))
  File "/usr/lib/python2.3/unittest.py", line 270, in fail
    raise self.failureException, msg
AssertionError: Unexpected exception catched

======================================================================
ERROR: test__setattr__ (__main__.TestTree)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tree_ut.py", line 29, in setUp
    self.fail(_("Unexpected exception catched"))
  File "/usr/lib/python2.3/unittest.py", line 270, in fail
    raise self.failureException, msg
AssertionError: Unexpected exception catched

======================================================================
ERROR: test__sgetitem__ (__main__.TestTree)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tree_ut.py", line 29, in setUp
    self.fail(_("Unexpected exception catched"))
  File "/usr/lib/python2.3/unittest.py", line 270, in fail
    raise self.failureException, msg
AssertionError: Unexpected exception catched

======================================================================
ERROR: test_tree (__main__.TestTree)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tree_ut.py", line 29, in setUp
    self.fail(_("Unexpected exception catched"))
  File "/usr/lib/python2.3/unittest.py", line 270, in fail
    raise self.failureException, msg
AssertionError: Unexpected exception catched

----------------------------------------------------------------------
Ran 4 tests in 0.006s

FAILED (errors=4)

这样的错误信息让人摸不着头脑，怎么会都是 Unexpected exception 呢？用：

def setUp(self):
    try:
        self.head = Tree('root', data='head.data', extra='head.extra')
        self.failUnlessEqual(self.head._Tree__node_value, 'root')
        self.failUnlessEqual(self.head.data, 'head.data')  #(1)
        self.failUnlessEqual(self.head.extra, 'head.extra')
    except Exception, ex:
        print 'DEBUG:', ex, ex.__class__
        seilf.fail(_("Unexpected exception catched"))

分析一下，可以发现是 #(1) 部分抛出的异常被 except 捕捉了，当然这里这个测试用例是不对的，但这也说明在 unittest 使用这种 try/except 语法是行不通的，因为这样就掩盖了真实的错误，让人搞不清方向。