can't delete from a dictionary in a loop

[Gary Herron]

Dan Upton wrote:
> This might be more information than necessary, but it's the best way I
> can think of to describe the question without being too vague.
>
> The task:
>
> I have a list of processes (well, strings to execute said processes)
> and I want to, roughly, keep some number N running at a time.  If one
> terminates, I want to start the next one in the list, or otherwise,
> just wait.
>
> The attempted solution:
>
> Using subprocess, I Popen the next executable in the list, and store
> it in a dictionary, with keyed on the pid:
> (outside the loop)
> procs_dict={}
>
> (inside a while loop)
> process = Popen(benchmark_exstring[num_started], shell=true)
> procs_dict[process.pid]=process
>
> Then I sleep for a while, then loop through the dictionary to see
> what's terminated.  For each one that has terminated, I decrement a
> counter so I know how many to start next time, and then try to remove
> the record from the dictionary (since there's no reason to keep
> polling it since I know it's terminated).  Roughly:
>
> for pid in procs_dict:
>   if procs_dict[pid].poll() != None
>    # do the counter updates
>    del procs_dict[pid]
>
> The problem:
>
> RuntimeError: dictionary changed size during iteration
>
> So, the question is: is there a way around this?  

Yes.  Create a list of keys, and loop through it:
pids = procs_dict.keys()
for pid in pids:
    if  procs_dict[pid].poll() != None
     # do the counter updates
     del procs_dict[pid]

Then the diction delete operation won't trip up the loop and its 
internal counter.



OR:  Create a list of things to delete while you are in the loop, and do 
the delete afterwards

deleteme = []
for pid in procs_dict:
  if procs_dict[pid].poll() != None
     # do the counter updates
     deleteme.append(pid)

for pid in deleteme:
   del procs_dict[pid]



OR: shred and rebuild the dictionary each time:

new_dict = {}
for pid,value in procs_dict.items():
   if value.poll() != None:
     # do the counter updates
      pass
    else:
      new_dict[pid] = value

procs_dict = new_dict



Gary Herron

  
> I know that I can
> just /not/ delete from the dictionary and keep polling each time
> around, but that seems sloppy and like it could keep lots of memory
> around that I don't need, since presumably the dictionary holding a
> reference to the Popen object means the garbage collector could never
> reclaim it.  Is the only reasonable solution to do something like
> append all of those pids to a list, and then after I've iterated over
> the dictionary, iterate over the list of pids to delete?
>
> (Also, from the implementation side, is there a reason the dictionary
> iterator can't deal with that?  If I was deleting from in front of the
> iterator, maybe, but since I'm deleting from behind it...)
> --
> http://mail.python.org/mailman/listinfo/python-list
>