Proper deletion of selected items during map iteration in for loop
Terry Reedy
tjreedy at udel.edu
Fri Apr 25 14:50:57 EDT 2014
On 4/25/2014 2:04 PM, Matthew Barnett wrote:
> On 2014-04-25 18:53, 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?
>>
> The other way is to build a new dictionary.
If you expect to delete more than half the keys *and* if there are no
other references to the dict, such that you need the particular object
mutated, this might be better.
> Actually, there's a third way. Iterate over a snapshot:
>
> for (k, v) in list(m.items()):
> if f(k):
> # do some processing of v and save result elsewhere
> del m[k]
Since a pre-snapshot of items or even just keys will be longer than a
list of keys to be deleted, I would stick with the latter.
--
Terry Jan Reedy
More information about the Python-list
mailing list