error handling

[Steven D'Aprano]

On Thu, 10 Aug 2006 16:59:56 -0700, Farshid Lashkari wrote:

> Chris wrote:
>> But sometimes you can have too many of these statements in your
>> program, and it starts to get tangled and nasty looking. Is there a way
>> I can modify sys.error so that when the interpreter comes accross an
>> IndexError it prints "That number is way too big!" before it exits?
> 
> Hi,
> 
> Try placing the try/except block around the main function of your program:
> 
> if __name__ == '__main__':
>      try:
>          main()
>      except IndexError:
>          sys.exit("That number is way too big!")


That's broken. 

Imagine that somewhere in main() the following is called:

D = {"a": "apple", "b": "bicycle", "c": "cat"}
print D["aardvark"]

Your code now prints "That number is way too big!". That's not good.

try...except blocks should, as a general rule, cover only the smallest
amount of code that they need to.

One possible solution is to use a custom class:

# untested
class MyList(list):
    def __getitem__(self, i):
        if i >= len(self):
            raise IndexError("That index is too big!")
        super(list, self).__getitem__(i)

Another is a helper function:

def safe_getindex(somelist, i):
    if i >= len(somelist):
        raise IndexError("That index is too big!")
    return somelist[i]


You can of course re-write them to use a try...except block instead of
testing the index.


-- 
Steven D'Aprano