5.4 如何关闭异常自动关联上下文？
================================

.. image:: http://image.iswbm.com/20200804124133.png

当你在处理异常时，由于处理不当或者其他问题，再次抛出另一个异常时，往外抛出的异常也会携带原始的异常信息。

就像这样子。

.. code:: python

   try:
       print(1 / 0)
   except Exception as exc:
       raise RuntimeError("Something bad happened")

从输出可以看到两个异常信息

.. code:: python

   Traceback (most recent call last):
     File "demo.py", line 2, in <module>
       print(1 / 0)
   ZeroDivisionError: division by zero

   During handling of the above exception, another exception occurred:

   Traceback (most recent call last):
     File "demo.py", line 4, in <module>
       raise RuntimeError("Something bad happened")
   RuntimeError: Something bad happened

如果在异常处理程序或 finally
块中引发异常，默认情况下，异常机制会隐式工作会将先前的异常附加为新异常的
``__context__``\ 属性。这就是 Python 默认开启的自动关联异常上下文。

如果你想自己控制这个上下文，可以加个 from 关键字（\ ``from``
语法会有个限制，就是第二个表达式必须是另一个异常类或实例。），来表明你的新异常是直接由哪个异常引起的。

.. code:: python

   try:
       print(1 / 0)
   except Exception as exc:
       raise RuntimeError("Something bad happened") from exc

输出如下

.. code:: python

   Traceback (most recent call last):
     File "demo.py", line 2, in <module>
       print(1 / 0)
   ZeroDivisionError: division by zero

   The above exception was the direct cause of the following exception:

   Traceback (most recent call last):
     File "demo.py", line 4, in <module>
       raise RuntimeError("Something bad happened") from exc
   RuntimeError: Something bad happened

当然，你也可以通过\ ``with_traceback()``\ 方法为异常设置上下文\ ``__context__``\ 属性，这也能在\ ``traceback``\ 更好地显示异常信息。

.. code:: python

   try:
       print(1 / 0)
   except Exception as exc:
       raise RuntimeError("bad thing").with_traceback(exc)

最后，如果我想彻底关闭这个自动关联异常上下文的机制？有什么办法呢？

可以使用 ``raise...from None``\ ，从下面的例子上看，已经没有了原始异常

.. code:: python

   $ cat demo.py
   try:
       print(1 / 0)
   except Exception as exc:
       raise RuntimeError("Something bad happened") from None
   $
   $ python demo.py
   Traceback (most recent call last):
     File "demo.py", line 4, in <module>
       raise RuntimeError("Something bad happened") from None
   RuntimeError: Something bad happened
   (PythonCodingTime)