我正在使用pdb调试名为a.py的程序
def f(x) :
x / x
def g(x) :
try :
f(x)
except Exception as e :
assert 0
g(0)
当我使用python3 -m pdb a.py运行程序时，程序在assert 0行停止，并且得到以下错误信息：
Traceback (most recent call last):
File "/tmp/a.py", line 6, in g
f(x)
File "/tmp/a.py", line 2, in f
x / x
ZeroDivisionError: division by zero
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/usr/lib64/python3.6/pdb.py", line 1667, in main
pdb._runscript(mainpyfile)
File "/usr/lib64/python3.6/pdb.py", line 1548, in _runscript
self.run(statement)
File "/usr/lib64/python3.6/bdb.py", line 434, in run
exec(cmd, globals, locals)
File "<string>", line 1, in <module>
File "/tmp/a.py", line 11, in <module>
g(0)
File "/tmp/a.py", line 9, in g
assert 0
AssertionError
和堆栈是（使用bt命令显示）：
(Pdb) bt
/usr/lib64/python3.6/pdb.py(1667)main()
-> pdb._runscript(mainpyfile)
/usr/lib64/python3.6/pdb.py(1548)_runscript()
-> self.run(statement)
/usr/lib64/python3.6/bdb.py(434)run()
-> exec(cmd, globals, locals)
<string>(1)<module>()->None
/tmp/a.py(11)<module>()->None
-> g(0)
> /tmp/a.py(9)g()
-> assert 0
(Pdb)
问题是，我不能仅使用x / x和up到函数f来调试down，因为我的堆栈在g函数处结束。
如何在异常中调试此类异常？异常内的异常又如何呢？
答案 0 :(得分：3)
tl;dr： 即使您已经进入外部异常的事后调试，您也可以调试内部异常。操作方法如下：
pdb 进入交互模式（在 interact 提示中输入 pdb）。import pdb, sys; pdb.post_mortem(sys.last_value.__context__.__traceback__)
__context__ 替换为 __cause__；如果嵌套更深，还可以附加更多 __context__ 或 __cause__。sys.last_value 替换为 sys.exc_info()[1]。如果您不确定，请在继续之前检查异常值。 （感谢 @The Doctor 在评论中指出这一点）pdb 会话，允许您调试内部异常。以下是对为什么这项工作的详细解释。在深入研究解决方案之前，我将首先解释一些相关概念：
链式异常
这里的“异常中的异常”被称为chained exceptions。异常可以显式或隐式链接：
>>>: try:
...: raise ZeroDivisionError
...: except Exception as inner_exc:
...: raise ValueError # implicit chaining
...:
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-6-1ae22e81c853> in <module>
1 try:
----> 2 raise ZeroDivisionError
3 except Exception as inner_exc:
ZeroDivisionError:
During handling of the above exception, another exception occurred:
ValueError Traceback (most recent call last)
<ipython-input-6-1ae22e81c853> in <module>
2 raise ZeroDivisionError
3 except Exception as inner_exc:
----> 4 raise ValueError # implicit chaining
ValueError:
>>>: try:
...: raise ZeroDivisionError
...: except Exception as inner_exc:
...: raise ValueError from inner_exc # explicit chaining
...:
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-5-63c49fcb10a2> in <module>
1 try:
----> 2 raise ZeroDivisionError
3 except Exception as inner_exc:
ZeroDivisionError:
The above exception was the direct cause of the following exception:
ValueError Traceback (most recent call last)
<ipython-input-5-63c49fcb10a2> in <module>
2 raise ZeroDivisionError
3 except Exception as inner_exc:
----> 4 raise ValueError from inner_exc # explicit chaining
ValueError:
如果我们将外部异常捕获为 outer_exc，那么我们可以通过 outer_exc.__cause__（如果显式链接）或 outer_exc.__context__（如果隐式链接）检查内部异常。
事后调试
使用 python -m pdb 运行脚本允许 Python 调试器在出现异常时进入 post-mortem debugging 模式。 “验尸”在这里的意思是“在异常发生之后”。您可以通过运行 %debug 魔法从 IPython 控制台或在 Jupyter 笔记本中执行相同的操作。
如果您有权访问回溯对象，也可以手动进入事后调试模式。幸运的是，回溯对象作为 __traceback__ 属性存储在异常对象本身上：
>>> try:
... raise ZeroDivisionError:
... except Exception as e:
... # Variable `e` is local to this block, so we store it in another variable
... # to extend its lifetime.
... exc = e
>>> import pdb
>>> pdb.post_mortem(exc.__traceback__)
> <ipython-input-8-e5b5ed89e466>(2)<module>()
-> raise ZeroDivisionError
(Pdb)
调试链式异常
现在我们可以尝试调试链式异常了！假设我们已经处于外部异常的事后调试模式。我们需要做的是：
pdb.post_mortem()。
我们的工作如下：# First, enter interactive mode to execute commands.
(Pdb) interact
*interactive*
# The current exception is stored in `sys.exc_info()`. This gives back a tuple
# of (exception type, exception value, traceback).
>>> import sys
>>> sys.exc_info()
(<class 'AssertionError'>, AssertionError(), <traceback object at 0x10c683e00>)
>>> sys.exc_info()[1]
AssertionError()
# In our case, the inner exception is implicitly chained. Access it through
# the `__context__` attribute.
>>> sys.exc_info()[1].__context__
ZeroDivisionError('division by zero')
# Get its traceback, and enter post-mortem debugging.
>>> sys.exc_info()[1].__context__.__traceback__
<traceback object at 0x10c683c80>
>>> import pdb
>>> pdb.post_mortem(sys.exc_info()[1].__context__.__traceback__)
> /Volumes/hrt/trunk-3/a.py(2)f()
-> x / x
(Pdb)
给你！您现在可以使用普通的 pdb 命令调试内部异常，例如遍历堆栈或检查局部变量。
答案 1 :(得分：1)
如我所见，那么您有几个选择。您可以将try-except移至f(x)，在g() try-except中打印一个描述性错误并放入pdb，或者在记录友好错误消息后重新引发异常。我不喜欢依靠未捕获的异常进行调试（选项3），因此选项1和2如下所示：
'''Option 1'''
def f(x) :
try:
x / x
except Exception as e:
print('An error occurred, entering pdb shell')
import pdb; pdb.set_trace()
def g(x) :
f(x)
g(0)
输出：
$ python throw.py
An error occurred, entering pdb shell
--Return--
> /home/wholevinski/so_test/throw.py(6)f()->None
-> import pdb; pdb.set_trace()
(Pdb) x
0
或：
'''Option 2'''
def f(x):
x / x
def g(x):
try:
f(x)
except Exception as e:
print('f({}) raised with error "{}". Entering pdb shell.'.format(x, str(e)))
import pdb; pdb.set_trace()
g(0)
输出：
$ python throw.py
f(0) raised with error division by zero. Entering pdb shell.
--Return--
> /home/wholevinski/so_test/throw.py(9)g()->None
-> import pdb; pdb.set_trace()
(Pdb) x
答案 2 :(得分：1)
实际上重新引发异常（ wholevinski的answer中的选项3）解决了我的问题，因为它不需要我修改函数f。 这是代码：
'''Option 3'''
def f(x) :
x -= 1
x / x
def g(x) :
try :
for i in range(x, 0, -1) :
print(f(i))
except Exception as e :
raise e
g(10)
堆栈的Pdb输出：
(Pdb) bt
/usr/lib64/python3.6/pdb.py(1667)main()
-> pdb._runscript(mainpyfile)
/usr/lib64/python3.6/pdb.py(1548)_runscript()
-> self.run(statement)
/usr/lib64/python3.6/bdb.py(434)run()
-> exec(cmd, globals, locals)
<string>(1)<module>()->None
/tmp/a.py(13)<module>()->None
-> g(10)
/tmp/a.py(11)g()
-> raise e
/tmp/a.py(9)g()
-> print(f(i))
> /tmp/a.py(4)f()
-> x / x
(Pdb)