Modify dict/set during iteration?

Steven D'Aprano steve at
Fri Oct 30 04:59:10 CET 2009

On Thu, 29 Oct 2009 19:02:01 -0700, metal wrote:

> I used this quickndirty way, any good idea to solve this problem?

It's not a problem that wants solving, it's a feature that wants paying 
attention to.

As a general rule, you shouldn't modify data structures while you're 
iterating over them, unless the data structure is advertised as safe to 
modify while being iterated over, or you can do so in a way that is 
guaranteed to be safe. When it comes to dicts and sets, neither of those 
conditions hold.

Why do you want to do this? It seems like a lot of effort to avoid a 
simple, straight-forward idiom:

make a copy of the dict or set
iterate over the copy, making changes to the original

What are you trying to accomplish by modifying objects while iterating 
over them?

> def miter(iterable):
>         except RuntimeError, e:
>             # Not sure if there were any other RuntimeError
>             if 'changed size during iteration' in e.message:
>                 continue

That is not safe. There's no guarantee that the error message won't 
change, even between one minor version to another, or between one Python 
implementation and another. If you insist on making such an unsafe test, 
at the very least be as conservative in what you expect as possible:

if 'changed' in e.message and 'size' in e.message:

and hope that nobody runs your code with internationalised error messages.


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