dictionary size changed during iteration

[Peter Otten]

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']