Python: eliminating stack traces into library code?

Question

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 sta

Answer 1

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 "", line 1, in 
  File "", line 3, in f
  File "", 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!-).