dictionary size changed during iteration

[Peter Otten]

Peter Otten wrote:

> 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):

Hm, I see you are using has_key() which is 2.x-only and invalidates my 
explanation :(

>>                  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']