[Tutor] Callbacks and exception handling

[Steven D'Aprano]

On Fri, 13 Aug 2010 11:56:31 am Pete wrote:
> Hi,
>
> I've been writing some code which uses callbacks. I have not used
> callbacks much before, but it does not seem too difficult so far.
>
> One thing though - I noticed that when an exception is raised in the
> callback function, that exception doesn't actually "show up" in the
> calling program.
>
> Two questions:
>
> 1) Theoretically - why is this? I guess I've seen callback functions
> a little like subroutines, therefore it seems intuitive that an
> exception would be propagated up to the calling routine.

They are, unless the calling function explicitly catches and subpresses 
the exception.


> 2) How can you catch an exception in the callback function?

The same way you catch an exception anywhere: with a try...except block.

If you are writing a callback function, and want to catch your own 
exception:


def callback():
    try:
        do_stuff_here
    except (KeyError, ValueError, TypeError):  # or whatever
        pass



If you are writing the calling function:


def caller(arg, callback):
    do_something_with(arg)
    try:
        callback()
    except (AttributeError, RecursionError):  # or whatever
        pass


By the way, don't be tempted to write a bare except clause:

try:
  ...
except:
  ...

There are very few reasons for such a thing, as they are too broad and 
catch too many things, masking errors, preventing user keyboard 
interrupts, etc. Avoid them.




-- 
Steven D'Aprano