slightly easier way to debug from the exception handler

[leorochael]

BPO	1106316
Nosy	@facundobatista, @etrepum

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/facundobatista'
closed_at = <Date 2008-03-08.16:51:10.237>
created_at = <Date 2005-01-20.22:06:51.000>
labels = ['type-feature', 'library']
title = 'slightly easier way to debug from the exception handler'
updated_at = <Date 2008-03-08.16:51:10.236>
user = 'https://bugs.python.org/leorochael'

bugs.python.org fields:

activity = <Date 2008-03-08.16:51:10.236>
actor = 'facundobatista'
assignee = 'facundobatista'
closed = True
closed_date = <Date 2008-03-08.16:51:10.237>
closer = 'facundobatista'
components = ['Library (Lib)']
creation = <Date 2005-01-20.22:06:51.000>
creator = 'leorochael'
dependencies = []
files = []
hgrepos = []
issue_num = 1106316
keywords = []
message_count = 5.0
messages = ['54351', '54352', '54353', '63062', '63404']
nosy_count = 3.0
nosy_names = ['facundobatista', 'leorochael', 'bob.ippolito']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue1106316'
versions = []

[leorochael]

When interactively developing/debugging a program it'd
be nice if we could throw the user into the pdb prompt
straight from the exception handler.

Currently, pdb.pm() only works outside of the exception
handler, after "sys.last_traceback" has already been
set, which doesn't happen inside the "except:" clause.
The alternative is to use:

try:
something...
except:
import sys, pdb
pdb.post_mortem(sys.exc_info()[2])

I propose, as a convenience to the programmer, that the
first parameter to pdb.post_mortem() be made optional,
defaulting to None. In this case, it'd automatically
use the value of sys.exc_info()[2] in place of it's
otherwise mandatory first parameter. The example above
would be reduced to:

try:
something...
except:
import pdb;pdb.post_mortem()

alternatively, we could make it so that if
"sys.last_traceback" is not set "pdb.pm()" would pick
up sys.exc_info()[2] instead...

[etrepum]

Logged In: YES
user_id=139309

Why put this in pdb.post_mortem() if we can just put it in pdb.pm()?
pdb.pm() seems to already be for this purpose (except it currently only
works from the interactive interpreter as you mention).

[leorochael]

Logged In: YES
user_id=200267

I don't have any particular reason to prefer post_mortem()
to pm().

The knowledgeable Python maintainers surely are better
equiped than I am to know if pm() looking beyond
sys.last_traceback is a worse break of assumptions than
post_mortem() having it's only parameter being optional or
the other way around :-)

[facundobatista]

If there's no objections, I'd make the traceback parameter optional in
the post_mortem method, making it default to sys.exc_info()[2].

Thanks!

[facundobatista]

Added this functionality in r61312.