如何在python中调试导致后续异常的堆栈跟踪？

Question

Python（和 ipython）具有非常强大的事后调试功能，允许在回溯的每个范围内进行变量检查和命令执行。up/down 调试器命令允许更改最终异常的堆栈跟踪的帧，但是__cause__按照raise ... from ... 语法定义的那个异常呢？

Python 3.7.6 (default, Jan  8 2020, 13:42:34) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.11.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: def foo(): 
   ...:     bab = 42 
   ...:     raise TypeError 
   ...:                                                                                                                                      

In [2]: try: 
   ...:     foo() 
   ...: except TypeError as err: 
   ...:     barz = 5 
   ...:     raise ValueError from err 
   ...:                                                                                                                                      
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-dd046d7cece0> in <module>
      1 try:
----> 2     foo()
      3 except TypeError as err:

<ipython-input-1-da9a05838c59> in foo()
      2     bab = 42
----> 3     raise TypeError
      4 

TypeError: 

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

ValueError                                Traceback (most recent call last)
<ipython-input-2-dd046d7cece0> in <module>
      3 except TypeError as err:
      4     barz = 5
----> 5     raise ValueError from err
      6 

ValueError: 

In [3]: %debug                                                                                                                               
> <ipython-input-2-dd046d7cece0>(5)<module>()
      2     foo()
      3 except TypeError as err:
      4     barz = 5
----> 5     raise ValueError from err
      6 

ipdb> barz                                                                                                                                   
5
ipdb> bab                                                                                                                                    
*** NameError: name 'bab' is not defined
ipdb> down                                                                                                                                   
*** Newest frame
ipdb> up                                                                                                                                     
*** Oldest frame

有没有办法bab从调试器访问？

编辑：我意识到事后调试不仅仅是 ipython 和 ipdb 的一个功能，它实际上是 vanilla pdb 的一部分。以上也可以通过将代码放入脚本中testerr.py，运行python -m pdb testerr.py运行来复现continue。错误后，它说

Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program

并在同一地点提供调试器。

Answer 1

您可以使用该with_traceback(tb)方法来保留原始异常的回溯：

try: 
    foo()
except TypeError as err:
    barz = 5
    raise ValueError().with_traceback(err.__traceback__) from err

请注意，我已更新代码以引发异常实例而不是异常类。

这是iPython 中的完整代码片段：

In [1]: def foo(): 
   ...:     bab = 42 
   ...:     raise TypeError() 
   ...:                                                                                                                                                         

In [2]: try: 
   ...:     foo() 
   ...: except TypeError as err: 
   ...:     barz = 5 
   ...:     raise ValueError().with_traceback(err.__traceback__) from err 
   ...:                                                                                                                                                         
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-a5a6d81e4c1a> in <module>
      1 try:
----> 2     foo()
      3 except TypeError as err:

<ipython-input-1-ca1efd1bee60> in foo()
      2     bab = 42
----> 3     raise TypeError()
      4 

TypeError: 

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

ValueError                                Traceback (most recent call last)
<ipython-input-2-a5a6d81e4c1a> in <module>
      3 except TypeError as err:
      4     barz = 5
----> 5     raise ValueError().with_traceback(err.__traceback__) from err
      6 

<ipython-input-2-a5a6d81e4c1a> in <module>
      1 try:
----> 2     foo()
      3 except TypeError as err:
      4     barz = 5
      5     raise ValueError().with_traceback(err.__traceback__) from err

<ipython-input-1-ca1efd1bee60> in foo()
      1 def foo():
      2     bab = 42
----> 3     raise TypeError()
      4 

ValueError: 

In [3]: %debug                                                                                                                                                  
> <ipython-input-1-ca1efd1bee60>(3)foo()
      1 def foo():
      2     bab = 42
----> 3     raise TypeError()
      4 

ipdb> bab                                                                                                                                                       
42
ipdb> u                                                                                                                                                         
> <ipython-input-2-a5a6d81e4c1a>(2)<module>()
      1 try:
----> 2     foo()
      3 except TypeError as err:
      4     barz = 5
      5     raise ValueError().with_traceback(err.__traceback__) from err

ipdb> u                                                                                                                                                         
> <ipython-input-2-a5a6d81e4c1a>(5)<module>()
      2     foo()
      3 except TypeError as err:
      4     barz = 5
----> 5     raise ValueError().with_traceback(err.__traceback__) from err
      6 

ipdb> barz                                                                                                                                                      
5

编辑 - 另一种劣等方法

解决@user2357112supportsMonica 的第一条评论，如果您希望避免在日志中多次转储原始异常的回溯，可以使用raise from None. 但是，正如@user2357112supportsMonica 的第二条评论所述，这隐藏了原始异常的消息。在您不是事后调试而是检查打印的回溯的常见情况下，这尤其成问题。

try: 
    foo()
except TypeError as err:
    barz = 5
    raise ValueError().with_traceback(err.__traceback__) from None

这是iPython 中的代码片段：

In [4]: try: 
   ...:     foo() 
   ...: except TypeError as err: 
   ...:     barz = 5 
   ...:     raise ValueError().with_traceback(err.__traceback__) from None    
   ...:                                                                                                                                                         
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-b090fb9c510e> in <module>
      3 except TypeError as err:
      4     barz = 5
----> 5     raise ValueError().with_traceback(err.__traceback__) from None
      6 

<ipython-input-6-b090fb9c510e> in <module>
      1 try:
----> 2     foo()
      3 except TypeError as err:
      4     barz = 5
      5     raise ValueError().with_traceback(err.__traceback__) from None

<ipython-input-2-ca1efd1bee60> in foo()
      1 def foo():
      2     bab = 42
----> 3     raise TypeError()
      4 

ValueError: 

In [5]: %debug                                                                                                                                                  
> <ipython-input-2-ca1efd1bee60>(3)foo()
      1 def foo():
      2     bab = 42
----> 3     raise TypeError()
      4 

ipdb> bab                                                                                                                                                       
42
ipdb> u                                                                                                                                                         
> <ipython-input-6-b090fb9c510e>(2)<module>()
      1 try:
----> 2     foo()
      3 except TypeError as err:
      4     barz = 5
      5     raise ValueError().with_traceback(err.__traceback__) from None

ipdb> u                                                                                                                                                         
> <ipython-input-6-b090fb9c510e>(5)<module>()
      3 except TypeError as err:
      4     barz = 5
----> 5     raise ValueError().with_traceback(err.__traceback__) from None
      6 

ipdb> barz                                                                                                                                                      
5

Raisingfrom None是必需的，否则链接将隐式完成，将原始异常附加为新异常的__context__属性。请注意，这与__cause__显式完成链接时设置的属性不同。

In [6]: try: 
   ...:     foo() 
   ...: except TypeError as err: 
   ...:     barz = 5 
   ...:     raise ValueError().with_traceback(err.__traceback__) 
   ...:                                                                                                                                                         
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-5-ee78991171cb> in <module>
      1 try:
----> 2     foo()
      3 except TypeError as err:

<ipython-input-2-ca1efd1bee60> in foo()
      2     bab = 42
----> 3     raise TypeError()
      4 

TypeError: 

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
<ipython-input-5-ee78991171cb> in <module>
      3 except TypeError as err:
      4     barz = 5
----> 5     raise ValueError().with_traceback(err.__traceback__)
      6 

<ipython-input-5-ee78991171cb> in <module>
      1 try:
----> 2     foo()
      3 except TypeError as err:
      4     barz = 5
      5     raise ValueError().with_traceback(err.__traceback__)

<ipython-input-2-ca1efd1bee60> in foo()
      1 def foo():
      2     bab = 42
----> 3     raise TypeError()
      4 

ValueError: