Re: [Python-ideas] for-while statement

Feb. 19, 2014

      On Feb 19, 2014, at 5:34, Alejandro López Correa <alc@spika.net> wrote:
...
Hello,
I think adding an optional "WHILE" clause in "FOR" loops might be
useful sometimes (shorter code and/or improved readability):
for #VAR# in #SEQUENCE# while #WATCHDOG_EXPRESSION#:
 #CODE_BLOCK#
Examples:
keepRunning = True
for i in range(100) while keepRunning:
   keepRunning = do_some_work( i )
found = False
for candidate in sequence while not found:
   try:
       process( candidate )
       found = True
   except InvalidCandidate:
       pass
retryCount = 7
for i in range(1,1+retryCount) while resource.acquire() == FAIL:
   sleep( i**2 )
At the moment, I usually implement this either with ugly breaks:
for i in range(100):
   if not do_some_work( i ):
       break
The only reason you think they're "ugly" is that you're thinking in C terms, not Python terms. 

This version is shorter, it's more explicit, and it has fewer places for you or after code maintainer to screw up and create bugs. (And yes, even in a trivial case like this, I've seen people write keepRunnig = ...)

And this becomes even more apparent in the longer cases. You're relying on the fact that setting a flag _plus_ falling off the end of a loop (which isn't nearly as obvious or visible or explicit) equals break.
...
found = False
for candidate in sequence:
   try:
       process_candidate()
   except InvalidCandidate:
       pass
   else:
      found = True
      break
Here, you don't need found at all. I suspect you're not using for...else because C doesn't have it?
...
Or with while loops and counters (or counting-like expressions):
i = 1
while i <= retryCount and not resource.acquired:
   if resource.acquire() == FAIL:
       sleep( i**2 )
   i += 1
Of course, actual code tends to be more complex, with "keepRunning"
being modified in some branches of "IF" blocks, and there might be
nested loops where the exit condition for the outer one is set in the
inner loop. Compare these two examples:
found = False
for filePath in glob( '*.data' ):
   for line in open( filePath, 'rt' ):
       if line.startswith( '#' ):
           continue
       if handle( line ):
           found = True
           break
   if found:
       break
found = False
for filePath in glob( '*.data' ) while not found:
   for line in open( filePath, 'rt' ) while not found:
       if line.startswith( '#' ):
           continue
       if handle( line ):
           found = True
-- Alejandro
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Re: [Python-ideas] for-while statement

Andrew Barnert