can't delete from a dictionary in a loop

[Dan Upton]

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?  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...)