If/then style question

[Steve Holden]

On 12/16/2010 11:32 PM, Carl Banks wrote:
> On Dec 16, 2:56 pm, Ryan Kelly <r... at rfk.id.au> wrote:
>> On Thu, 2010-12-16 at 21:49 +0000, John Gordon wrote:
>>> (This is mostly a style question, and perhaps one that has already been
>>> discussed elsewhere.  If so, a pointer to that discussion will be
>>> appreciated!)
>>
>>> When I started learning Python, I wrote a lot of methods that looked like
>>> this:
>>
>>>   def myMethod(self, arg1, arg2):
>>>     if some_good_condition:
>>>       if some_other_good_condition:
>>>         if yet_another_good_condition:
>>>           do_some_useful_stuff()
>>>           exitCode = good1
>>>         else:
>>>           exitCode = bad3
>>>       else:
>>>         exitCode = bad2
>>>     else:
>>>       exitCode = bad1
>>>     return exitCode
>>
>>> But lately I've been preferring this style:
>>
>>>   def myMethod(self, arg1, arg2):
>>>     if some_bad_condition:
>>>       return bad1
>>>     elif some_other_bad_condition:
>>>       return bad2
>>>     elif yet_another_bad_condition:
>>>       return bad3
>>>     do_some_useful_stuff()
>>>     return good1
>>
>>> I like this style more, mostly because it eliminates a lot of indentation.
>>
>>> However I recall one of my college CS courses stating that "one entry,
>>> one exit" was a good way to write code, and this style has lots of exits.
>>
>>> Are there any concrete advantages of one style over the other?
>>
>> "one entry, one exit" has its good points, but it's *way* overquoted and
>> overused.
>>
>> Do you raise any exceptions? Do you call any functions that might raise
>> exceptions?  If so, you've got multiple exit points already.
>>
>> I think this style a lot more important in a language like C where you
>> have to be super-careful about cleaning up after yourself.  The single
>> exit point makes it easier to verify that all cleanup tasks have been
>> performed.  Assuming you're using "with" or "try-finally" then you just
>> don't need such guarantees in python.
> 
> Even without the cleanup issue, sometimes you want to edit a function
> to affect all return values somehow.  If you have a single exit point
> you just make the change there; if you have mulitple you have to hunt
> them down and change all of them--if you remember to.  I just got bit
> by that one.
> 
> It's a trade-off.  Readability and/or conciseness versus error
> robustness.  I tend to go for the former but mileage varies.
> 
Heaven forfend you should just wrap the existing function inside another
one which takes its name, if all its returns need to be altered.

regards
 Steve
-- 
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