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.

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.

Answer 2

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) 

Answer 3

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

__exception__

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