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1.5.2 for: else:

Gordon McMillan gmcm at hypernet.com
Wed Jul 28 17:38:24 EDT 1999 
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
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