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Date: Fri, 25 Apr 2014 19:04:40 +0100
From: Matthew Barnett <mrabarnett@mrabarnett.plus.com>
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Subject: Re: Proper deletion of selected items during map iteration in for loop
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Xref: csiph.com comp.lang.python:70610

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.

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]