Re: Modify dict/set during iteration?

metal wrote:
Steven D'Aprano wrote:
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.

--
Steven

Yes the idoim rulz. I confused my self.

Maybe my real goal is the following:

def miter(iterable):
for x in tuple(iterable):
if x in iterable:
yield x


That works, though I'd change the parameter name to something to indicate that it's a set or dictionary, not a general iterable.

As someone else pointed out, you can't use 'in' on a generator, for example.

Assuming a dict, what happens if a new item created in the dictionary, but reusing an old key? It won't cause an exception, but you run the risk of either processing some data twice, or some not-at-all. Whether that matters or not depends on your use-case. Sometimes you need to copy the keys, and sometimes you need to copy the keys and their values.

DaveA

.

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