Laszlo Nagy wrote:
> Given this iterator:
>
> class SomeIterableObject(object):
> ....
> ....
>
> def __iter__(self):
> ukeys = self.updates.keys()
> for key in ukeys:
> if self.updates.has_key(key):
> yield self.updates[key]
> for rec in self.inserts:
> yield rec
> ....
> ....
>
> How can I get this exception:
>
> RuntimeError: dictionary changed size during iteration
>
>
> It is true that self.updates is being changed during the iteration. But
> I have created the "ukeys" variable solely to prevent this kind of
> error. Here is a proof of correctness:
>
>>>> d = {1:1,2:2}
>>>> k = d.keys()
>>>> del d[1]
>>>> k
> [1, 2]
>>>> k is d.keys()
> False
>
> So what is wrong with this iterator? Why am I getting this error message?
The keys() method which used to return a list in 2.x was changed in 3.x to
return a view object and to become more or less the equivalent of the old
dict.iterkeys():
>>> d = dict(a=1)
>>> keys = d.keys()
>>> keys
dict_keys(['a'])
>>> for k in keys:
... d["b"] = 42
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration
>>> keys
dict_keys(['a', 'b'])
You now have to create the list explicitly to avoid the error:
>>> d = dict(a=1)
>>> keys = list(d.keys())
>>> for k in keys:
... d["b"] = 42
...
>>> d
{'a': 1, 'b': 42}
>>> keys
['a']
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