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I've been trying to get a specific (and most likely useless) effect on the traceback when I raise an error.
I would like to hide the raise Exception("Message") of the traceback inside my function and instead have my trace indicate the function call that triggered the raise inside the function
def crash(something):
if something > 200:
raise ValueError("Value too large !")
if __name__ == '__main__':
crash(500)
Normally generates :
Traceback (most recent call last):
File "file.py", line 7, in <module>
crash(500)
File "file.py", line 3, in crash
raise ValueError("Value too large !")
ValueError: Value too large !
Instead I would like to have this :
Traceback (most recent call last):
File "file.py", line 7, in <module>
crash(500)
ValueError: Value too large !
I find it clearer for the user trying to use the module because it clearly shows where the problem is as otherwise he could potentially think the module itself is at fault
I tried to find solution that would "remove" the last call from the traceback but it always had "side effects" like showing part of the code trying to remove the last trace call INSIDE the traceback itself, making it even more confusing. Sometimes it would also repeat the the traceback over what I really want.
For example using this :
def crash():
frame = sys._getframe(1)
tb = types.TracebackType(None, frame, frame.f_lasti, frame.f_lineno)
raise ValueError("Wrong value").with_traceback(tb)
Prints this :
Traceback (most recent call last):
File "file.py", line 34, in <module>
crash()
File "file.py", line 9, in crash
raise ValueError("Wrong value").with_traceback(tb)
File "file.py", line 34, in <module>
crash()
ValueError: Wrong value
I also tried using another function to create the trace myself but ended up with a weird behavior
def crash():
raise exception_no_raise(ValueError("Wrong value"))
def exception_no_raise(exc: Exception):
tb = None
depth = 0
while True:
try:
sys._getframe(depth)
depth += 1
except ValueError:
break
# for i in range(depth-1, 1, -1):
# frame = sys._getframe(i)
# tb = types.TracebackType(tb, frame, frame.f_lasti, frame.f_lineno)
# traceback.print_tb(tb)
# print(file=sys.stderr)
frame = sys._getframe(depth-1)
tb = types.TracebackType(tb, frame, frame.f_lasti, frame.f_lineno)
return exc.with_traceback(tb)
Prints :
Traceback (most recent call last):
File "file.py", line 32, in <module>
crash()
File "file.py", line 7, in crash
raise exception_no_raise(ValueError("Wrong value"))
File "file.py", line 32, in <module>
crash()
ValueError: Wrong value
Even tho when you use traceback.print_tb(tb) (before the return) you get the exact trace I want even tho using it to raise the exception doesn't print it :
File "file.py", line 34, in <module>
crash()
The last solution I found was to "strip" the last traceback from the tb.tb_next chain of traces after catching the exception and then re-raise it (but it showed the raise code anyway so ...)
I have difficulties to grasp how exactly traceback works and it seems that it changes drastically from version to version as I found code from Python 2 and 3 and also code the works in Python 3.8 and not before (related to the fact that you couldn't write to the tb.next of a trace before
Thanks for your help and clarifications !
So when I run this... the error is on this line bomb=pd.DataFrame(here,0) but the trace shows me a bunch of code from the pandas library to get to the error.
import traceback,sys
import pandas as pd
def error_handle(err_var,instance_name=None): #err_var list of variables, instance_name
print(traceback.format_exc())
a= sys._getframe(1).f_locals
for i in err_var: # selected var for instance
t= a[instance_name]
print i,"--->",getattr(t,i.split(".")[1])
here=['foo']
err_var = ['self.needthisone','self.constant2']
class test:
def __init__(self):
self.constant1 = 'hi1'
#self.constant2 = 'hi2'
#self.needthisone = ':)'
for i in err_var:
setattr(self, i.split('.')[1], None)
def other_function(self):
self.other_var=5
def testing(self):
self.other_function()
vars=[self.constant1,self.constant2]
try:
for i in vars:
bomb=pd.DataFrame(here,0)
except:
error_handle(err_var,'self')
t=test()
t.testing()
How do I suppress all that and have the error just look like this:
Traceback (most recent call last):
File "C:\Users\Jason\Google Drive\python\error_handling.py", line 34, in testing
bomb=pd.DataFrame(here,0)
TypeError: Index(...) must be called with a collection of some kind, 0 was passed
I just want what's relevant to me and the last line of code that I wrote which was bad.
This is the original:
Traceback (most recent call last):
File "C:\Users\Jason\Google Drive\python\error_handling.py", line 35, in testing
bomb=pd.DataFrame(here,0)
File "C:\Python27\lib\site-packages\pandas\core\frame.py", line 330, in __init__
copy=copy)
File "C:\Python27\lib\site-packages\pandas\core\frame.py", line 474, in _init_ndarray
index, columns = _get_axes(*values.shape)
File "C:\Python27\lib\site-packages\pandas\core\frame.py", line 436, in _get_axes
index = _ensure_index(index)
File "C:\Python27\lib\site-packages\pandas\core\indexes\base.py", line 3978, in _ensure_index
return Index(index_like)
File "C:\Python27\lib\site-packages\pandas\core\indexes\base.py", line 326, in __new__
cls._scalar_data_error(data)
File "C:\Python27\lib\site-packages\pandas\core\indexes\base.py", line 678, in _scalar_data_error
repr(data)))
TypeError: Index(...) must be called with a collection of some kind, 0 was passed
self.needthisone ---> None
self.constant2 ---> None
You can define how far back a traceback goes using the sys.traceback variable. If your code is only 3 levels deep, (a function in a class in a file), then you can define this appropriately with the code:
sys.tracebacklimit = 3
at the top of your file. BE CAREFUL WITH THIS: As you write more code, the portion that you've written will become deeper and deeper, and you may sometime soon find that an error is a result of something deeper in the traceback. As a general rule, I would avoid using the variable and just deal with a longer traceback for the time being.
Please, don't ever think about limiting the stack trace. It is very important.
Only at this moment, in this small example of yours, the error really is in your code.
But in an infinite other cases, an error could be triggered much deeper than that. It could be in the framework, or even out of the code whatsoever, like a configuration error, or it could be in the platform, like an Out of Memory error, etc.
The stack trace is there to help you. It lists all frames the compiler was executing, to give you all the info you need to understand what was going on.
I would highly encourage you not to limit your traceback output, because it is bad practice. You feel like there is too much info; but this is only because you already looked at it already and you know what error to look for.
In most cases, the problem may be hiding elsewhere. So there has to be a better way to achieve what you look for.
Why not wrap your function call in a try except clause and print the exception message? Take this scenario for example:
def f():
a = 0
i = 1
print i/a
def another_func():
print 'this is another func'
return f()
def higher_level_func():
print 'this is higher level'
return another_func()
if __name__ == '__main__':
try:
higher_level_func()
except Exception as e:
print 'caught the exception: {}-{}'.format(type(e)__name__, e.message)
When called, this is the output:
this is higher level
this is another func
caught the exception: ZeroDivisionError-integer division or modulo by zero
This prints only the relevant exception in your code, hiding any information about the traceback, but the traceback is still available and you can print it as well (just raise the exception from your except block).
Compared to this, if I remove the try except block:
this is higher level
this is another func
caught the exception: integer division or modulo by zero
Traceback (most recent call last):
File "test.py", line 17, in <module>
higher_level_func()
File "test.py", line 12, in higher_level_func
return another_func()
File "test.py", line 8, in another_func
return f()
File "test.py", line 4, in f
print i/a
ZeroDivisionError: integer division or modulo by zero
You better use this technique to capture the relevant exception, rather than limiting the tracebacks. If you want your program to stop, just add sys.exit(1) in the except block.
Having a raised exception I would like to jump into that frame. To explain better what I mean I wrote this mwe:
Assuming I have the following code:
from multiprocessing import Pool
import sys
# Setup debugger
def raiseDebugger(*args):
""" http://code.activestate.com/recipes/65287-automatically-start-the-
debugger-on-an-exception/ """
import traceback, pdb
traceback.print_exception(*args)
pdb.pm()
sys.excepthook = raiseDebugger
# Now start with the question
def faulty(i):
return 1 / i
with Pool() as pool:
pool.map(faulty, range(6))
which unsurprisingly leads to:
multiprocessing.pool.RemoteTraceback:
"""
Traceback (most recent call last):
File "/home/bin/conda/lib/python3.5/multiprocessing/pool.py", line 119, in worker
result = (True, func(*args, **kwds))
File "/home/bin/conda/lib/python3.5/multiprocessing/pool.py", line 44, in mapstar
return list(map(*args))
File "test2.py", line 19, in faulty
return 1 / i
ZeroDivisionError: division by zero
"""
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "test2.py", line 23, in <module>
pool.map(faulty, range(6))
File "/home/bin/conda/lib/python3.5/multiprocessing/pool.py", line 260, in map
return self._map_async(func, iterable, mapstar, chunksize).get()
File "/home/bin/conda/lib/python3.5/multiprocessing/pool.py", line 608, in get
raise self._value
ZeroDivisionError: division by zero
> /home/bin/conda/lib/python3.5/multiprocessing/pool.py(608)get()
-> raise self._value
(Pdb)
Now to debug the problem I would like to "jump" into the frame which originally raised the exception (ZeroDivisionError).
The original exception is still available under self._value complete with self._value.__traceback__.
The call that pm (or post_mortem) calls is from the value field of sys.exc_info, and the default invocation of post_mortem is done on the __traceback__ of that value. However if you want to get to the underlying object, you want to access its __context__ instead. Given this code example:
import pdb
import sys
import traceback
def top():
value = 1
raise Exception('this always fails')
def bottom():
try:
top()
except Exception as bot_ex:
x = {}
return x['nothing']
try:
bottom()
except Exception as main_ex:
pdb.post_mortem()
Running the code. The main_ex would be analogous to your self._value.
> /tmp/foo.py(14)bottom()
-> return x['nothing']
(Pdb) main_ex
KeyError('nothing',)
(Pdb) pdb.post_mortem(main_ex.__traceback__)
> /tmp/foo.py(14)bottom()
-> return x['nothing']
Note we have a new pdb prompt at the same location, which is where the exception was originally raised. Let's try it with __context__ if we need to go further up:
(Pdb) c
(Pdb) pdb.post_mortem(main_ex.__context__.__traceback__)
> /tmp/foo.py(7)top()
-> raise Exception('this always fails')
If needed, keep repeating until you get to the target context/traceback desired.
Now for the multiprocessing case, which I wasn't aware would have made this much difference, as the question implies something general (How can I “jump” into stackframe from exception?), but it turns out the specifics in multiprocessing made all the difference.
In Python 3.4 a workaround was done to just show that traceback as a string; due to how much stuff a traceback actually has, communicating all that proved to be difficult as discussed in the issue 13831 on the Python tracker, so instead a hack was done to bring a __cause__ attribute into the current exception, but it is no full __traceback__ as it just has the string representation of that, as I had suspected.
Anyway this is what would have happened:
(Pdb) !import pdb
(Pdb) !self._value.__cause__
RemoteTraceback('\n"""\nTraceback (most recent call last):...',)
(Pdb) !type(self._value.__cause__)
<class 'multiprocessing.pool.RemoteTraceback'>
(Pdb) !self._value.__cause__.__traceback__
(Pdb) !self._value.__cause__.__context__
So this isn't actually possible until they figure out how to bring all those states across process boundaries.
I tried to reproduce it with some simpler functions but didn't succeed. So the following code shows the relevant methods for a KeyError which get's thrown by our production servers, a lot.
class PokerGame:
...
def serialsNotFold(self):
return filter(lambda x: not self.serial2player[x].isFold(), self.player_list)
def playersNotFold(self):
return [self.serial2player[serial] for serial in self.serialsNotFold()]
...
And here is the Traceback.
Traceback (most recent call last):
File "/usr/lib/python2.6/dist-packages/pokernetwork/pokertable.py", line 945, in update
try: self.game.historyReduce()
File "/usr/lib/python2.6/dist-packages/pokerengine/pokergame.py", line 3949, in historyReduce
self.turn_history = PokerGame._historyReduce(self.turn_history,self.moneyMap())
File "/usr/lib/python2.6/dist-packages/pokerengine/pokergame.py", line 1323, in moneyMap
money = dict((player.serial,player.money) for player in self.playersNotFold())
File "/usr/lib/python2.6/dist-packages/pokerengine/pokergame.py", line 3753, in playersNotFold
return [self.serial2player[serial] for serial in self.serialsNotFold()]
KeyError: 21485L
self.player_list is a list with serials
self.serial2player is a dict which maps serials to Player objects
Now it shouldn't be possible that the KeyError is raised in playersNotFold because therefore the same error had to be raised in serialsNotFold, which it gets not.
I asked my 2 peers and the guys on #python but no one was able to even gues how this can happen.
If you need the full source: https://github.com/pokermania/poker-network/
EDIT:
The Problem was that we printed traceback.format_exc(limit=4) which limits from the top instead of the bottom. The last 2 calls where hidden, so it looked like playersNotFold raised the exception.
Here is a full trace.
Traceback (most recent call last):
File "/usr/lib/python2.7/pokernetwork/pokertable.py", line 704, in update
try: self.game.historyReduce()
File "/usr/lib/python2.7/pokerengine/pokergame.py", line 3953, in historyReduce
self.turn_history = PokerGame._historyReduce(self.turn_history,self.moneyMap())
File "/usr/lib/python2.7/pokerengine/pokergame.py", line 1327, in moneyMap
money = dict((player.serial,player.money) for player in self.playersNotFold())
File "/usr/lib/python2.7/pokerengine/pokergame.py", line 3757, in playersNotFold
return self.serial2player[serial] for serial in self.serialsNotFold()]
File "/usr/lib/python2.7/pokerengine/pokergame.py", line 3754, in serialsNotFold
return filter(lambda x: not self.serial2player[x].isFold(] self.player_list)
File "/usr/lib/python2.7/pokerengine/pokergame.py", line 3754, in <lambda>
return filter(lambda x: not self.serial2player[x].isFold(] self.player_list)
KeyError: 1521
I'm sorry for wasting your time :/
I guess you use threads and self.serial2player gets modified by a different thread.
When I get a runtime exception from the standard library, it's almost always a problem in my code and not in the library code. Is there a way to truncate the exception stack trace so that it doesn't show the guts of the library package?
For example, I would like to get this:
Traceback (most recent call last):
File "./lmd3-mkhead.py", line 71, in <module>
main()
File "./lmd3-mkhead.py", line 66, in main
create()
File "./lmd3-mkhead.py", line 41, in create
headver1[depotFile]=rev
TypeError: Data values must be of type string or None.
and not this:
Traceback (most recent call last):
File "./lmd3-mkhead.py", line 71, in <module>
main()
File "./lmd3-mkhead.py", line 66, in main
create()
File "./lmd3-mkhead.py", line 41, in create
headver1[depotFile]=rev
File "/usr/anim/modsquad/oses/fc11/lib/python2.6/bsddb/__init__.py", line 276, in __setitem__
_DeadlockWrap(wrapF) # self.db[key] = value
File "/usr/anim/modsquad/oses/fc11/lib/python2.6/bsddb/dbutils.py", line 68, in DeadlockWrap
return function(*_args, **_kwargs)
File "/usr/anim/modsquad/oses/fc11/lib/python2.6/bsddb/__init__.py", line 275, in wrapF
self.db[key] = value
TypeError: Data values must be of type string or None.
update: added an answer with the code, thanks to the pointer from Alex.
The traceback module in Python's standard library lets you emit error tracebacks in a way that accords to your liking, while an exception is propagating. You can use this power either in the except leg of a try/except statement, or in a function you've installed as sys.excepthook, which gets called if and when an exception propagates all the way; quoting the docs:
In an interactive session this happens
just before control is returned to the
prompt; in a Python program this
happens just before the program exits.
The handling of such top-level
exceptions can be customized by
assigning another three-argument
function to sys.excepthook.
Here's a simple, artificial example:
>>> import sys
>>> import traceback
>>> def f(n):
... if n<=0: raise ZeroDivisionError
... f(n-1)
...
>>> def excepthook(type, value, tb):
... traceback.print_exception(type, value, tb, 3)
...
>>> sys.excepthook = excepthook
>>> f(8)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in f
File "<stdin>", line 3, in f
ZeroDivisionError
as you see, without needing a try/except, you can easily limit the traceback to (for example) the first three levels -- even though we know by design that there were 9 nested levels when the exception was raised.
You want something more sophisticated than a simple limit on levels, so you'll need to call traceback.format_exception, which gives you a list of lines rather than printing it, then "prune" from that list the lines that are about modules you never want to see in your tracebacks, and finally emit the remaining lines (typically to sys.stderr, but, whatever!-).
Thanks to the pointer from Alex, here's teh codez:
def trimmedexceptions(type, value, tb, pylibdir=None, lev=None):
"""trim system packages from the exception printout"""
if pylibdir is None:
import traceback, distutils.sysconfig
pylibdir = distutils.sysconfig.get_python_lib(1,1)
nlev = trimmedexceptions(type, value, tb, pylibdir, 0)
traceback.print_exception(type, value, tb, nlev)
else:
fn = tb.tb_frame.f_code.co_filename
if tb.tb_next is None or fn.startswith(pylibdir):
return lev
else:
return trimmedexceptions(type, value, tb.tb_next, pylibdir, lev+1)
import sys
sys.excepthook=trimmedexceptions
# --- test code ---
def f1(): f2()
def f2(): f3()
def f3():
import xmlrpclib
proxy = xmlrpclib.ServerProxy('http://nosuchserver')
proxy.f()
f1()
Which yields this stack trace:
Traceback (most recent call last):
File "./tsttraceback.py", line 47, in <module>
f1()
File "./tsttraceback.py", line 40, in f1
def f1(): f2()
File "./tsttraceback.py", line 41, in f2
def f2(): f3()
File "./tsttraceback.py", line 45, in f3
proxy.f()
gaierror: [Errno -2] Name or service not known
The Traceback library is probably what you want. Here's one example that might help:
import traceback
try:
your_main()
except:
lines = traceback.format_exc()
print lines[:lines.find('File "/usr')]
(This obviously won't work if there's an exception outside the library, and might not exactly fit your needs, but it's one way of using the traceback library)
Put an unqualified try...except at the top of your code (ie: in your "main") or set sys.excepthook. You can then format the stack trace however you'd like.