2.11 The traceback Module

The traceback module in Example 2-18 allows you to print exception tracebacks inside your programs, just like the interpreter does when you don't catch an exception yourself.

Example 2-18. Using the traceback Module to Print a Traceback

File: traceback-example-1.py

# note! importing the traceback module messes up the
# exception state, so you better do that here and not
# in the exception handler
import traceback

try:
    raise SyntaxError, "example"
except:
    traceback.print_exc()

Traceback (innermost last):
  File "traceback-example-1.py", line 7, in ?
SyntaxError: example

Example 2-19 uses the StringIO module to put the traceback in a string.

Example 2-19. Using the traceback Module to Copy a Traceback to a String

File: traceback-example-2.py

import traceback
import StringIO

try:
    raise IOError, "an i/o error occurred"
except:
    fp = StringIO.StringIO()
    traceback.print_exc(file=fp)
    message = fp.getvalue()

    print "failure! the error was:", repr(message)

failure! the error was: 'Traceback (innermost last):\012  File
"traceback-example-2.py", line 5, in ?\012IOError: an i/o error
occurred\012'

To format the traceback in a nonstandard way, use the extract_tb function to convert a traceback object to a list of stack entries, as Example 2-20 demonstrates.

Example 2-20. Using the traceback Module to Decode a Traceback Object

File: traceback-example-3.py

import traceback
import sys

def function():
    raise IOError, "an i/o error occurred"

try:
    function()
except:
    info = sys.exc_info()
    for file, lineno, function, text in traceback.extract_tb(info[2]):
        print file, "line", lineno, "in", function
        print "=>", repr(text)
    print "** %s: %s" % info[:2]

traceback-example-3.py line 8 in ?
=> 'function()'
traceback-example-3.py line 5 in function
=> 'raise IOError, "an i/o error occurred"'
** exceptions.IOError: an i/o error occurred