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.

Answer 1

Answering to question 3, you can use:

raise B('second') from None 

Which will remove the exception A traceback.

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

Answer 2

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