Proper deletion of selected items during map iteration in for loop

[Terry Reedy]

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