Weird behavior removing elements from a list in a loop in Python -

i understand can't remove element list if iterating on list. i'm trying copy elements list not wish remove list, , replacing original list new list. here's relevant code:

while len(tokenlist) > 0:     # loop through tokenlist list      # reset updated token list , remove flag     updatedtokenlist = []     removeflag = false      token in tokenlist:          completionhash = aciserver.checktaskforcompletion(token)          # if completion hash not empty hash, parse information         if completionhash != {}:             # if find task has completed, remove list             if completionhash['status'] == 'finished' , completionhash['error'] == '':                 # task completed successfully, remove token list                 removeflag = true              elif completionhash['status'] == 'running' , completionhash['error'] == '':                 # task must still running                 print('task ' + completionhash['type'] + ' ' + token + ' has been running ' + completionhash['runtime'] + ' seconds.')              elif completionhash['status'] == 'queued':                 # task in queue                 print('task ' + completionhash['type'] + ' ' + token + ' queued in position ' + completionhash['queueposition'])              elif completionhash['status'] == 'not_found':                 # did not find task token, possible task hasn't been added yet                 print(completionhash['error'])              # if task still running, no change token list have occured          else:             # because server got rid of token after task completed             print('completion hash empty, went wrong.')              tokenlisterror.append(token)             removeflag = true          if not removeflag:             print('appending token updatedtokenlist')             updatedtokenlist.append(token)       print('length of tokenlist after removal loop: ' + str(len(updatedtokenlist)))      # wait time, proportional number of tasks left     checkinterval = len(updatedtokenlist) * checkintervalmultiplier      print('waiting ' + str(checkinterval) + ' seconds before checking again...')     print('tokens remaining: ' + str(len(updatedtokenlist)))      # replace original token list updated token list     tokenlist = updatedtokenlist      # wait while based on how many tokens remain     time.sleep(checkinterval) 

so point of update tokenlist new list. every time through loop, new tasks have finished , should not added updatedtokenlist. remaining task tokens , replaces original token list.

this not work. on first pass through, not add tokens updatedtokenlist though no tasks have yet completed. cannot figure out doing wrong. suggestions?

this gets lot easier if move logic function:

#this function should have more descriptive name follows  #project's api. def should_keep(token):     """returns true if token should kept"""     #do other stuff here.  possibly print stuff or whatever ...     ... 

now, can replace list simple list comprehension:

tokenlist = [ token token in tokenlist if should_keep(token) ] 

note haven't actually replaced list. old list still presumably have references hanging around. if want replace list in place though, it's no problem. use slice assignment:

tokenlist[:] = [ token token in tokenlist if should_keep(token) ]