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 ;)
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