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Python (and ipython) has very powerful post-mortem debugging capabilities, allowing variable inspection and command execution at each scope in the traceback. The up/down debugger commands allow changing frame for the stack trace of the final exception, but what about the __cause__ of that exception, as defined by the raise ... from ... syntax?
Python 3.7.6 (default, Jan 8 2020, 13:42:34)
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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
Is there a way to access bab from the debugger?
EDIT: I realized post-mortem debugging isn't just a feature of ipython and ipdb, it's actually part of vanilla pdb. The above can also be reproduced by putting the code into a script testerr.py and running python -m pdb testerr.py and running continue. After the error, it says
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
and gives a debugger at the same spot.
882
You can use the with_traceback(tb) method to preserve the original exception's traceback:
try:
foo()
except TypeError as err:
barz = 5
raise ValueError().with_traceback(err.__traceback__) from err
Note that I have updated the code to raise an exception instance rather than the exception class.
Here is the full code snippet in iPython:
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
Addressing @user2357112supportsMonica's first comment, 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 second comment states, this hides the original exception's message. This is particularly problematic in the common case where you're not post-mortem debugging but rather inspecting a printed traceback.
try:
foo()
except TypeError as err:
barz = 5
raise ValueError().with_traceback(err.__traceback__) from None
Here is the code snippet in iPython:
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
Raising from None is required since otherwise the chaining would be done implicitly, attaching the original exception as the new exception’s __context__ attribute. Note that this differs from the __cause__ attribute which is set when the chaining is done explicitly.
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:
Yoel answer works and should be your go-to procedure, but if the trace is a bit harder to debug, you may instead use the trace module.
The trace module will print out each instruction executed, line by line. There is a catch, though. Standard library and package calls will also be traced, and this likely means that the trace will be flooded with code that is not meaningful.
To avoid this behavior, you may pass the --ignore-dir argument with the location of your Python library and site packages folder.
Run python -m site to find the locations of your site packages, then call trace with the following arguments:
python -m trace --trace --ignore-dir=/usr/lib/python3.8:/usr/local/lib/python3.8/dist-packages main.py args
Replacing the ignore-dir with all folders and the main.py args with a script location and arguments.
You may also use the Trace module directly in your code if you want to run a certain function, refer to this example extracted from 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")
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