Proper deletion of selected items during map iteration in for loop

Sat Apr 26 17:30:57 EDT 2014

Charles Hixson wrote:
> What is the proper way to delete selected items during iteration of a
> map? What I want to do is:
>> for (k, v) in m.items():
> if f(k):
> # do some processing of v and save result elsewhere
> del m[k]
>> But this gives (as should be expected):
> RuntimeError: dictionary changed size during iteration
> In the past I've accumulated the keys to be deleted in a separate list,
> but this time there are likely to be a large number of them, so is there
> some better way?

It just struck me that you can store the keys to be deleted as values in the 
same dict. That way you need no extra memory:
def delete_items(d, keys):
 keys = iter(keys)
 try:
 first = prev = next(keys)
 except StopIteration:
 return
 for key in keys:
 d[prev] = prev = key
 d[prev] = first
 key = first
 while True:
 key = d.pop(key)
 if key is first:
 break
if __name__ == "__main__":
 import string
 data = dict(zip(range(10), string.ascii_lowercase))
 print("original data:", data)
 print("removing odd items...")
 delete_items(data, keys=(k for k in data if k % 2))
 print("modified data:", data)
 print("delete no items...")
 delete_items(data, [])
 print("modified data:", data)
 print("delete a single item (6)...")
 delete_items(data, [6])
 print("modified data:", data)
 print("delete all items...")
 delete_items(data, data)
 print("modified data:", data)
While I think I am a genius* in practice this approach will probably not be 
the most effective.
(*) (Un)fortunately that feeling never lasts longer than a few minutes ;)