Get last exception in pdb

Question

Is there a way to examine the last exception when in pdb/before entering pdb? (Using python 2.7.5).

Immediately (yes, I enter no other commands at all) after an exception being raised in my code, I do sys.exc_info(); this just results in (None, None, None). At this point, I can do pdb.pm(), and pdb starts at the point that the exception is raised.

I'd like to be able to examine this exception object (it is not stored in a variable before being raised).

There is nothing obviously helpful in http://docs.python.org/2/library/pdb.html or http://docs.python.org/2/library/sys.html

Edit: I know about set_trace. I'd like to examine the exception before I modify the code.

Washedout

Get reference to the current exception has an answer that works for my use case: getting the last thrown exception when stepping through with pdb. – Mirisola

Answer 1

You can use sys.last_value:

>>> no_such_var
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'no_such_var' is not defined
>>> import sys
>>> sys.last_value
NameError("name 'no_such_var' is not defined",)
>>> sys.last_value.args
("name 'no_such_var' is not defined",)

---

>>> no_such_var
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'no_such_var' is not defined
>>> import pdb, sys
>>> pdb.set_trace()
--Return--
> <stdin>(1)<module>()->None
(Pdb) sys.last_value
NameError("name 'no_such_var' is not defined",)

NOTE: This solution is not perfect. The value is set when an exception is not handled and the interpreter prints an error message and a stack traceback. For example, if the exception is caught using try .. except .., sys.last_value is not set.

Dhobi

I get: *** AttributeError: 'module' object has no attribute 'last_value' – Selfconscious

@claymation, That attribute is not set if there was no exception. Execute a statement that causes an exception. For example: 1/0 – Dhobi

@Dhobi -> print 1 / 0, (Pdb) n, ZeroDivisionError: 'integer division or modulo by zero', (Pdb) import sys, (Pdb) print sys.last_value, *** AttributeError: 'module' object has no attribute 'last_value' - it does not exist (python 2.6) – Polyvalent

Per the documentation, that value is only set if the exception was fatal. – Mirisola

@zneak, Could you share the documentation url? – Dhobi

@falsetru, you literally posted it yourself: docs.python.org/2/library/sys.html#sys.last_value – Mirisola

@zneak, But, there's no mentation about fatal. – Dhobi

@falsetru, what is your interpretation of "These three variables are not always defined; they are set when an exception is not handled and the interpreter prints an error message and a stack traceback"? – Mirisola

@zneak, Ah, sorry for my poor English reading. Added that part in the answer. Thank you. – Dhobi

I appreciate the attention that this is getting; but note that it still doesn't work when tracing through pdb. Try this and see if you can get the exception from the (pdb) prompt. Alternatively, set_trace() before 1/0, hit n and see if you can get the exception there. – Mirisola

@zneak, Isn't try .. except ... handing exception ? – Dhobi

Yes, but what if my catch clause was like (ZeroDivisionError, IOError, ValueError) and I wanted to get the complete message without adding a temporary ... as exc from pdb? As OP says: "I'd like to examine the exception before I modify the code." – Mirisola

@zneak, How about import sys, then !sys.exc_info()? (didn't put this in the answer because OP already tried sys.exc_info() and got (None, None, None). – Dhobi

@Dhobi My comment was using pdb in a script, not the shell, similar to zneak's pastebin after your comment @-ing me – Polyvalent

Answer 2

You can retrieve the latest exception in pdb/ipdb via:

__exception__

The above is actually a tuple of the (exception, message).

Runabout

"NameError: name '__exception__' is not defined" – Pouncey

same, python 3.8.x – Atrioventricular

Works for me on Google Colab. Thanks! – Marissamarist

This worked for me when I started pdb first and then triggered the exception by stepping through the code. It did not work for me when the exception is triggered before starting pdb (e.g. with pdb.post_mortem). – Subclass

Answer 3

Is this what you are looking for?

import pdb
try:
    1/0
except Exception as err:
    pdb.set_trace()

---

% test.py
--Return--
> /home/unutbu/pybin/test.py(8)<module>()->None
-> pdb.set_trace()
(Pdb) err
ZeroDivisionError('integer division or modulo by zero',)
(Pdb) quit

---

If you do not want to modify the code where the exception originates, you could instead redefine sys.excepthook:

import pdb
import sys
def excepthook(type, value, traceback):
    pdb.set_trace()
sys.excepthook = excepthook

1/0

---

% test.py
--Return--
> /home/unutbu/pybin/test.py(7)excepthook()->None
-> pdb.set_trace()
(Pdb) type
<type 'exceptions.ZeroDivisionError'>
(Pdb) value
ZeroDivisionError('integer division or modulo by zero',)
(Pdb) traceback
<traceback object at 0xb774f52c>
(Pdb) 

Furfural

Answer 4

You can run the script through pdb via python -m pdb -c continue script.py. It will enter post-mortem debugging on an uncaught exception and drop you in the pdb interface. Here you can examine sys.exc_info() in order to get the exception. For example:

$ echo "1 / 0" > script.py 
$ python -m pdb -c continue script.py 
Traceback (most recent call last):
  [...]
  File "/tmp/script.py", line 1, in <module>
    1 / 0
ZeroDivisionError: division by zero
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> /tmp/script.py(1)<module>()
-> 1 / 0
(Pdb) !import sys
(Pdb) p sys.exc_info()
(<class 'ZeroDivisionError'>, ZeroDivisionError('division by zero'), <traceback object at 0x7f3adcf09148>)
(Pdb) interact
*interactive*
>>> import sys
>>> sys.exc_info()
(<class 'ZeroDivisionError'>, ZeroDivisionError('division by zero'), <traceback object at 0x7f3adcf09148>)

Bespangle

I only figured this out like two weeks ago myself, after using python for 12 years. It's actually a wonderful solution when available. – Washedout

(<class 'AttributeError'>, AttributeError("'Pdb' object has no attribute 'do_sys'"), <traceback object at 0x10dabf800>) on python 3.10 – Labroid

^ For the above I ended up having to use !sys.exc_info() which apparently executes on a different frame. – Labroid

Answer 5

I bumped into this post, but none of the answers did what I needed, which was to repeat all of the error information (including the traceback) that was spewed before the pdb.pm() step in the question. Here's what worked for me in the end:

Add the following lines to your .pdbrc file:

import sys
import traceback
alias rethrow traceback.print_exception(sys.last_type, sys.last_value, sys.last_traceback)

Now, next time you're in a pdb environment, you can just type rethrow, and it will repeat all the error information from before you had typed pdb.pm().

Subak

This is the only solution that worked for me (python 3.8) – Munition