Raising exceptions when an exception is already present in Python 3

Question

What happens to my first exception (A) when the second (B) is raised in the following code?

class A(Exception): pass
class B(Exception): pass

try:
    try:
        raise A('first')
    finally:
        raise B('second')
except X as c:
    print(c)

If run with X = A I get:

Traceback (most recent call last):
  File "raising_more_exceptions.py", line 6, in 
    raise A('first')
__main__.A: first

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "raising_more_exceptions.py", line 8, in 
    raise B('second')
__main__.B: second

But if X = B I get:

second

Questions

1. Where did my first exception go?
2. Why is only the outermost exception catchable?
3. How do I peel off the outermost exception and reraise the earlier exceptions?

Update0

This question specifically addresses Python 3, as its exception handling is quite different to Python 2.

question from:https://stackoverflow.com/questions/6278426/raising-exceptions-when-an-exception-is-already-present-in-python-3

Answer 1

The 'causing' exception is available as c.__context__ in your last exception handler. Python is using this information to render a more useful traceback. Under Python 2.x the original exception would have been lost, this is for Python 3 only.

Typically you would use this to throw a consistent exception while still keeping the original exception accessible (although it's pretty cool that it happens automatically from an exception handler, I didn't know that!):

try:
    do_something_involving_http()
except (URLError, socket.timeout) as ex:
    raise MyError('Network error') from ex

More info (and some other pretty useful things you can do) here: http://docs.python.org/3.3/library/exceptions.html