Confused compare function :)

Steven D'Aprano steve+comp.lang.python at pearwood.info
Thu Dec 6 12:47:50 CET 2012


On Thu, 06 Dec 2012 09:49:26 +0100, Bruno Dupuis wrote:

> The point is Exceptions are made for error handling, not for normal
> workflow. 

That's certainly not the case in Python. Using exceptions for flow 
control is a standard part of the language.

IndexError and StopIteration are used to detect the end of lists or 
iterators in for loops.

GeneratorExit is used to request that generators exit.

SysExit is used to exit the interpreter.


There is nothing wrong with using exceptions for flow control in 
moderation.


> I hate when i read that for example:
> 
>     try:
>         do_stuff(mydict[k])
>     except KeyError:
>         pass
> 
> (loads of them in many libraries and frameworks) instead of:
> 
>     if k in mydict:
>         do_stuff(mydict[k])
> 
> Note that the performances are better with the latter.

Not so. Which one is faster will depend on how often you expect to fail. 
If the keys are nearly always present, then:

try:
    do_stuff(mydict[k])
except KeyError:
    pass

will be faster. Setting up a try block is very fast, about as fast as 
"pass", and faster than "if k in mydict".

But if the key is often missing, then catching the exception will be 
slow, and the "if k in mydict" version may be faster. It depends on how 
often the key is missing.


[...]
> It depends also on the context, I'd be more 'permissive' a short script
> than into a large program, framework, or lib, for the very reason it's
> easy to know all code interactions.
> 
> In my coder life, i spent more time debugging silently swallowed
> exceptions than logging abnormal behaviours.


That's fine. I agree with you about not silently swallowing errors. Where 
I disagree is that you said "never ever", which is an exaggeration. 
Remember that exceptions are not always errors.

Problem: take a list of strings, and add up the ones which are integers, 
ignoring everything else.

Solution:

total = 0
for s in list_of_strings:
    try:
        total += int(s)
    except ValueError:
        pass  # Not a number, ignore it.


Why would you want to log that? It's not an error, it is working as 
designed. I hate software that logs every little thing that happens, so 
that you cannot tell what's important and what isn't.



-- 
Steven



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