If/then style question

[Carl Banks]

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.


Carl Banks