Confused compare function :)

peter pjmakey2 at gmail.com
Thu Dec 6 12:55:16 CET 2012


On 12/06/2012 08:47 AM, Steven D'Aprano wrote:
> 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.
>
>
>
Is perfectly right to use try catch for a flow control.
Just think in something more complex like this.

    try:
             self._conn = MySQLdb.connect(host=host,
                 user=user,
                 passwd=passwd,
                 db=db)
     except:
             logging.info("Error de conexion con la base de datos")
             inform(subject = 'Db down on app %s' % app, body=sbody)

Or maybe something like this.

try:
             cursor.execute(sqli, data)
             self._conn.commit()
except:
             try:
                 self._conn.rollback()
                 cursor.execute(sqli, data)
                 self._conn.commit()
             except Exception, e:
                 pass
                 # print e
                 # logging.info('ERROR en la insercion %s' % e)

This is pretty dumb, but is a valid example, on what you can do with try 
catch



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