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"for" with "else"?

Stephen Horne $$$$$$$$$$$$$$$$$ at $$$$$$$$$$$$$$$$$$$$.co.uk
Wed Oct 1 22:32:04 EDT 2003 
On Wed, 01 Oct 2003 03:31:10 GMT, "Andrew Dalke"
<adalke at mindspring.com> wrote:

>In my search, I didn't see any examples which were
>better done with exceptions -- and since (string) exceptions
>existed in the language for a long time (from the start
>I would imagine and definitely pre-1.3, which is
>about when I started), I find it hard to believe that
>your statement reflects what really happened.

You are quite right - I think I mentioned already in another post that
I'd completely forgotten the break-on-found idiom until I read Michael
Gearys post (I don't always read a thread fully before I reply to
stuff).

I still think that exception-based approaches are far from painful for
this kind of stuff. However, with your example from binhex.py...

  for c in data:
      if not c.isspace() and (c<' ' or ord(c) > 0x7f):
          break
  else:
      finfo.Type = 'TEXT'

The cleanest way to eliminate the 'break' and 'else' is probably as
follows...

  try:
    c = data.next ()
    while c.isspace() or (c<' ' or ord(c) > 0x7f) :  c = data.next ()
  except StopIteration :
    finfo.Type = 'TEXT'

Or possibly...

  try :
    ifilter (lambda c : c in string.printable, data).next ()
  except StopIteration :
    finfo.Type = 'TEXT'

But these approaches both have a major failing - they don't express
the intention well.

Actually, there is a very clean and expressive approach that I'd use
if I had access to my own library stuff...

  if mylib.exists (lambda c : c in string.printable, data) :
    finfo.Type = 'TEXT'

with, in 'mylib'...

  def exists (pred, seq) :
    for i in seq :
      if pred(i) : return True
    return False

...which is, of course, cheating as that return is just as
unstructured as a break - but I don't mind cheating too much when it
is localised in a trivial library function.


-- 
Steve Horne

steve at ninereeds dot fsnet dot co dot uk
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