Python exception handling - line number

I'm using python to evaluate some measured data. Because of many possible results it is difficult to handle or possible combinations. Sometimes an error happens during the evaluation. It is usually an index error because I get out of range from measured data.

It is very difficult to find out on which place in code the problem happened. It would help a lot if I knew on which line the error was raised. If I use following code:

try:
    result = evaluateData(data)
except Exception, err:
    print ("Error: %s.\n" % str(err))

Unfortunately this only tells me that there is and index error. I would like to know more details about the exception (line in code, variable etc.) to find out what happened. Is it possible?

Thank you.

Solution 1:

Solution, printing filename, linenumber, line itself and exception descrpition:

import linecache
import sys

def PrintException():
    exc_type, exc_obj, tb = sys.exc_info()
    f = tb.tb_frame
    lineno = tb.tb_lineno
    filename = f.f_code.co_filename
    linecache.checkcache(filename)
    line = linecache.getline(filename, lineno, f.f_globals)
    print 'EXCEPTION IN ({}, LINE {} "{}"): {}'.format(filename, lineno, line.strip(), exc_obj)


try:
    print 1/0
except:
    PrintException()

Output:

EXCEPTION IN (D:/Projects/delme3.py, LINE 15 "print 1/0"): integer division or modulo by zero

Solution 2:

To simply get the line number you can use sys, if you would like to have more, try the traceback module.

import sys    
try:
    [][2]
except IndexError:
    print("Error on line {}".format(sys.exc_info()[-1].tb_lineno))

prints:

Error on line 3

Example from the traceback module documentation:

import sys, traceback

def lumberjack():
    bright_side_of_death()

def bright_side_of_death():
    return tuple()[0]

try:
    lumberjack()
except IndexError:
    exc_type, exc_value, exc_traceback = sys.exc_info()
    print "*** print_tb:"
    traceback.print_tb(exc_traceback, limit=1, file=sys.stdout)
    print "*** print_exception:"
    traceback.print_exception(exc_type, exc_value, exc_traceback,
                              limit=2, file=sys.stdout)
    print "*** print_exc:"
    traceback.print_exc()
    print "*** format_exc, first and last line:"
    formatted_lines = traceback.format_exc().splitlines()
    print formatted_lines[0]
    print formatted_lines[-1]
    print "*** format_exception:"
    print repr(traceback.format_exception(exc_type, exc_value,
                                          exc_traceback))
    print "*** extract_tb:"
    print repr(traceback.extract_tb(exc_traceback))
    print "*** format_tb:"
    print repr(traceback.format_tb(exc_traceback))
    print "*** tb_lineno:", exc_traceback.tb_lineno

Solution 3:

I use the traceback which is simple and robust:

import traceback

try:
    raise ValueError()
except:
    print(traceback.format_exc())

Out:

Traceback (most recent call last):
  File "catch.py", line 4, in <module>
    raise ValueError()
ValueError

Solution 4:

The simplest way is just to use:

import traceback
try:
    <blah>
except IndexError:
    traceback.print_exc()

or if using logging:

import logging
try:
    <blah>
except IndexError as e:
    logging.exception(e)