Re-raise exception with a different type and message, preserving existing information

Python 3 introduced exception chaining (as described in PEP 3134). This allows, when raising an exception, to cite an existing exception as the “cause”:

try:
    frobnicate()
except KeyError as exc:
    raise ValueError("Bad grape") from exc

The caught exception (exc, a KeyError) thereby becomes part of (is the “cause of”) the new exception, a ValueError. The “cause” is available to whatever code catches the new exception.

By using this feature, the __cause__ attribute is set. The built-in exception handler also knows how to report the exception's “cause” and “context” along with the traceback.

---

In Python 2, it appears this use case has no good answer (as described by Ian Bicking and Ned Batchelder). Bummer.

You can use sys.exc_info() to get the traceback, and raise your new exception with said traceback (as the PEP mentions). If you want to preserve the old type and message, you can do so on the exception, but that's only useful if whatever catches your exception looks for it.

For example

import sys

def failure():
    try: 1/0
    except ZeroDivisionError, e:
        type, value, traceback = sys.exc_info()
        raise ValueError, ("You did something wrong!", type, value), traceback

Of course, this is really not that useful. If it was, we wouldn't need that PEP. I'd not recommend doing it.

You could create your own exception type that extends whichever exception you've caught.

class NewException(CaughtException):
    def __init__(self, caught):
        self.caught = caught

try:
    ...
except CaughtException as e:
    ...
    raise NewException(e)

But most of the time, I think it would be simpler to catch the exception, handle it, and either raise the original exception (and preserve the traceback) or raise NewException(). If I were calling your code, and I received one of your custom exceptions, I'd expect that your code has already handled whatever exception you had to catch. Thus I don't need to access it myself.

Edit: I found this analysis of ways to throw your own exception and keep the original exception. No pretty solutions.