I'm trying to do something similar to the following:
try:
1/0
except ZeroDivisionError as e:
import ipdb; ipdb.set_trace()
When I drop into the debugger, I'd like for the exception instance e
to be in my local scope. However, if I run this script, I find that this is not the case:
Kurts-MacBook-Pro-2:Scratch kurtpeek$ python debug_exception.py
--Return--
None
> /Users/kurtpeek/Documents/Scratch/debug_exception.py(4)<module>()
2 1/0
3 except ZeroDivisionError as e:
----> 4 import ipdb; ipdb.set_trace()
ipdb> dir()
['__annotations__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__return__', '__spec__', 'ipdb']
ipdb> e
*** NameError: name 'e' is not defined
Why is e
not defined? I'm currently using print
statements to find out the attributes of e
, but I feel this should be possible to do interactively.
ipdb.set_trace()
doesn't quite trigger IPDB immediately. It triggers on the next trace event , which in your case, is after the except
block ends.
Python 3 deletes the e
variable at the end of the except
block, to break traceback reference cycles . Unfortunately for you, that happens before IPDB can trigger.
One hacky workaround you could use would be to add another line after set_trace
, so IPDB triggers on the 'line'
event:
try:
1/0
except ZeroDivisionError as e:
import ipdb
ipdb.set_trace()
workaround = True
Another option would be to use post-mortem debugging, which doesn't need to wait for a trace event:
try:
1/0
except ZeroDivisionError as e:
import ipdb
ipdb.post_mortem()
Post-mortem debugging has a number of important differences from regular debugging, though. It'll put you in the (usually dead) stack frame where the exception occurred, rather than the stack frame where the post_mortem
call occurred. Those happen to be the same frame in your example, but they usually won't be. Having access to the stack frame where the exception was raised is pretty nice, and you can still navigate to the frame where the exception was caught (but no further, due to Python's unusual traceback system), but it's still a major difference.
Also, you can't step in post-mortem mode. Trying to run next
or step
will exit debugging.
Actually you can use post_mortem
to access the traceback context
import ipdb; ipdb.post_mortem()
ipdb> e
ZeroDivisionError('division by zero',)
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