python - sys.exc_info() 是如何工作的？

Question

sys.exc_info() 的行为在 python docs 中有描述。和 SO和 SO作为:

• 在 except block 或由 except block 调用的方法中，描述异常的三元组
• 在其他地方，三元组 None 值

那么为什么这个 nosetest 会失败呢？

def test_lang_stack(self):
    try:
        self.assertEquals((None,None,None), sys.exc_info()) # no exception
        a = 5 / 0
    except:
        self.assertNotEquals((None,None,None), sys.exc_info())  # got exception
    else:
        self.fail('should not get here')
    #finally:
    #    self.assertEquals((None,None,None), sys.exc_info()) # already handled, right?
    self.assertEquals((None,None,None), sys.exc_info())  # already handled, right?

它在最后一行失败了。如果您取消注释 finally block ，它将在那里失败。

我确实看到，如果我将所有这些放在一个方法中并从另一个方法调用，那么调用方法就不会看到异常。 exc_info 值似乎仍设置为抛出异常的方法的末尾，即使在处理异常之后也是如此。

我在 OSX 上使用 python2.7。

Answer 1

在回答你的问题之前，我必须解释一下 sys.exc_info() 决定了你程序中最新/最近的异常:-

Your program is basically a series of function calls, with caller function calling called. Thus a call stack/execution stack is formed, where an entry is pushed for each function, being called. This entry is known as stack frame in python. Thus, when in your program, sys.exc_info() is invoked, it uses the following sequence to determining latest exception. It begins from the current stack frame, which is the function from where you are invoking sys.exc_info(). If the current stack frame is not handling/had not handled an exception, the information is taken from the calling stack frame, or its caller, and so on until a stack frame is found that is handling/had handled an exception . Here, “handling an exception” is defined as “executing or having executed an except clause.” For any stack frame, only information about the most recently handled exception is accessible.

现在，开始你的代码，

def test_lang_stack(self):
    try:
        self.assertEquals((None,None,None), sys.exc_info()) # no exception
        a = 5 / 0
    except:
        self.assertNotEquals((None,None,None), sys.exc_info())  # got exception
    else:
        self.fail('should not get here')
    #finally:
    #    self.assertEquals((None,None,None), sys.exc_info()) # already handled, right?
    self.assertEquals((None,None,None), sys.exc_info())  # already handled, right?

按照上面解释的过程，sys.exc_info() 总是会发现最近在您的函数中处理的异常。因此，除非您显式调用 sys.exec_clear()，否则它永远不会是 Nones 的元组。

希望对你有帮助。