[Python-ideas] for-while statement

[Alejandro López Correa]

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

found = False
for candidate in sequence:
    try:
        process_candidate()
    except InvalidCandidate:
        pass
    else:
       found = True
       break


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