Question

I have a code sample below, where I want to catch some exception:

in the main function:

try:
    ...
    do_something()
except some_exception,e:
    do_some_other_thing

in the do_something function

try:
    ...
except some_exception,e:
    do_some_other_thing

So when I was testing it, I noticed that the exception was handled twice (once in do_somthing(), and once in the main function), is my observation accurate?

Is there a way to catch the exception that are only not captured by its function? Because there are two scenarios that I want to catch and they are somewhat wrapped into the same exception handling class(i.e. some_exception)

Answer 1

Your observation is inaccurate; there must be 2 places that raise some_exeption, or you are explicitly re-raising it.

Once an exception has correctly been caught, it will not be caught by other handlers higher up in the stack.

This is easiest shown with a little demonstration:

>>> try:
...     try:
...         raise AttributeError('foo')
...     except AttributeError as e:
...         print 'caught', e
... except AttributeError as e:
...     print 'caught the same exception again?', e
... 
caught foo

Only the inner except handler was invoked. If, on the other hand, we re-raise the exception, you’ll see both handlers print a message:

>>> try:
...     try:
...         raise AttributeError('foo')
...     except AttributeError as e:
...         print 'caught', e
...         raise e
... except AttributeError as e:
...     print 'caught the same exception again?', e
... 
caught foo
caught the same exception again? foo

As such, there is no need to have to handle ‘exceptions that are not caught by the function’; you will only need to handle exceptions that were not already caught earlier.