如何调试导致 python 中后续异常的堆栈跟踪？

Question

Python（和ipython）具有非常强大的post-mortem 调试功能，允许在回溯中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堆栈跟踪的帧，但是 raise ... from ... 语法定义的那个异常的 __cause__ 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

有没有办法从调试器访问 bab？

编辑：我意识到 post-mortem 调试不仅仅是 ipython 和 ipdb 的一个特性，它实际上是 vanilla pdb 的一部分。也可以通过将代码放入脚本 testerr.py 和运行 python -m pdb testerr.py 和运行 continue 来重现上述内容。报错后显示

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

并在同一位置提供调试器。

Answer 1

您可以使用 with_traceback(tb) method 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 class。

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 - 另一种较差的方法

寻址 @user2357112supportsMonica 的 , if you wish to avoid multiple dumps of the original exception's traceback in the log, it's possible to raise from None. However, as @user2357112supportsMonica's 状态，这隐藏了原始异常的消息。在您不是 post-mortem 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

需要引发 from None，否则链接将完成 implicitly，将原始异常附加为新异常的 __context__ 属性。请注意，这与明确完成链接时设置的 __cause__ 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:

Answer 2

Yoel 答案有效，应该是您的首选程序，但如果跟踪难以调试，您可以改用 trace 模块。

跟踪模块将逐行打印出执行的每条指令。不过有一个问题。还将跟踪标准库和程序包调用，这可能意味着跟踪将充斥着无意义的代码。

为避免此行为，您可以将 --ignore-dir 参数与 Python 库和站点包文件夹的位置一起传递。

运行 python -m site 找到您的站点包的位置，然后使用以下参数调用跟踪：

python -m trace --trace --ignore-dir=/usr/lib/python3.8:/usr/local/lib/python3.8/dist-packages main.py args

将 ignore-dir 替换为所有文件夹，将 main.py args 替换为脚本位置和参数。

如果你想运行某个功能，你也可以直接在你的代码中使用Trace模块，参考这个例子摘自https://docs.python.org/3.0/library/trace.html:

import sys
import trace

# create a Trace object, telling it what to ignore, and whether to
# do tracing or line-counting or both.
tracer = trace.Trace(
    ignoredirs=[sys.prefix, sys.exec_prefix],
    trace=0,
    count=1)

# run the new command using the given tracer
tracer.run('main()')

# make a report, placing output in /tmp
r = tracer.results()
r.write_results(show_missing=True, coverdir="/tmp")

Answer 3

我还找到了一种无需修改底层源代码即可执行此操作的方法 - 只需 post-mortem 调试器中的运行命令即可。

我从看到你可以直接从回溯实例中获取局部变量。

(Pdb) ll
  1  -> def foo():
  2         bab = 42
  3         raise TypeError
  4   
  5     try:
  6         foo()
  7     except TypeError as err:
  8         barz = 5
  9  >>     raise ValueError from err
 10
(Pdb) err # not sure why err is not defined
*** NameError: name 'err' is not defined
(Pdb) import sys
(Pdb) sys.exc_info()
(<class 'AttributeError'>, AttributeError("'Pdb' object has no attribute 'do_sys'"), <traceback object at 0x107cb5be0>)
(Pdb) err = sys.exc_info()[1].__context__
(Pdb) err # here we go
ValueError()
(Pdb) err.__cause__
TypeError()
(Pdb) err.__traceback__.tb_next.tb_next.tb_next.tb_next.tb_frame.f_locals['barz']
5
(Pdb) err.__cause__.__traceback__.tb_next.tb_frame.f_locals['bab']
42

Answer 4

我很惊讶没有简单的方法来沿着异常链移动，并在所需级别进行调试。所以我写了一个小的 IPython 魔法 %chain，让它变得简单。只需将其放入您的 iPython 启动目录（例如 ~/.ipython/profile_default/startup）

如何调试导致 python 中后续异常的堆栈跟踪？

How to debug the stack trace that causes a subsequent exception in python?

python

ipython

python-3.x

pdb

ipdb

编辑 - 另一种较差的方法