Re: [Python-ideas] for-while statement

Feb. 19, 2014


      On 2014-02-19, at 14:34 , 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):
It seems judicious application of itertools can do the job.
...
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
for i in takewhile(do_some_work, range(100)):
    pass
...
found = False
for candidate in sequence:
   try:
       process_candidate()
   except InvalidCandidate:
       pass
   else:
      found = True
      break
for candidate in sequence:
    try:
        process_candidate()
    except InvalidCandidate:
        pass
    else:
        # found
        break
else:
    # not found
...
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
# itertools's missing piece
flatmap = lambda fn, *it: chain.from_iterable(imap(fn, *it))

lines = flatmap(open, iglob('*.data'), repeat('rb')
non_comments = ifilter(lambda line: not line.startswith('#'), lines)
matches = ifilter(handle, non_comments)

match = next(matches, None)

Alternatively the filters could be replaced by

matches =(line for line in lines
          if not line.startswith('#')
          if handle(line))

Re: [Python-ideas] for-while statement

Masklinn