1.5.2 for: else:

[Stidolph, David]

As I am understanding for .. else, the following

for window in self.windows:
  self.ProcessWindow(window)
else:
  print 'windows processed'

is the same as

for window is self.windows:
  self.ProcessWindow(window)
print 'windows processed'

and the only case where the else statement has any value over simply adding
code after the for loop would be if a break statement were executed - in
which case the else would NOT be called.  Personally, I would rather have
the else execute if none of the code within the for loop were executed.  It
would save a variable assignment/test as in:

executed = 0
for window in self.windows:
  executed = 1
  self.ProcessWindow(window)
if not executed:
  self.HandleProcessWindowNotCalled()

This forces the variable to be set numerous times (needless).

I'm with those that see else being an alternate execution (like the if)
rather than some sort of continuation.  I'm also with those that favor "if
it isn't broke, don't fix it".

-----Original Message-----
From: Gordon McMillan [mailto:gmcm at hypernet.com]
Sent: Wednesday, July 28, 1999 4:38 PM
To: python-list at cwi.nl
Subject: Re: 1.5.2 for: else:


William Tanksley wrote:
> On Tue, 27 Jul 1999 21:43:00 -0500, Gordon McMillan wrote:

[snip Billy's silly error and his entirely insufficient embarassment 
<wink>]

> However, you're wrong about the best way to detect a 2 in the list
> using this style.  To steal the same example:
> 
> try:
>  for x in [1,2,3]:
>   print x
>   if x == 2: raise neverMind
>  print "modern else clause here"
> except neverMind: print "found two"
> else:
>  print "another place to put the modern else clause"
> 
> Wow, TMTOWTDI.  Oops, did I say the wrong thing?
> 
> Of course, by demonstrating both ways to do it, I'm making the code
> more complex; but you can see that this solution solves the problem
> quite elegantly.

WIth an extra class, and 3 or 4 more lines of code, not to mention 
treading on the toes of all who think exceptions should only be used 
for exceptional conditions (or maybe it _is_ exceptional when 
something you wrote works <1e3 wink>).

> The place to use MY definition of 'else', OTOH, is when you're
> building a structure during the iteration, but the result of an
> empty loop is different than the initial value the variable has to
> have when going through the loop.
> 
> Here you go.  This example might convert a Python list into a Lisp
> tree-list -- but to imitate Lisp (okay, it's a toy problem, I'm
> cheating) an empty list produces NIL ~= None.
> 
> var = Node(None,None)
> for x in pyList:
>   var.car = x
>   var.cdr = Node()
> else:
>   var = None

Not a Lisper, but isn't what you're doing better done thusly:

def test1(lst):
 var = None
 for x in lst:
  var = (x, var)
 return var

print `test1([1,2,3])`

print `test1([])`
 
Produces:
(3, (2, (1, None)))
None
 
> >insufferably-sanctimoniously-y'rs
> 
> >- Gordon
> 
> unstoppably-confidantly y'rs
> 
> -- 
> -William "Billy" Tanksley

so-self-righteous-without-the-winks-I-might-almost-be-mistaken
 -for-that-other-GM-ly y'rs

- Gordon