iteritems in generator and delete in Python2, no errors?

Question

I have a very strange worries, with this way:

for k, v in ((k, v) for k, v in mydict.iteritems() if v.update):
    print k
    del mydict[k]

I know perfectly well that we should not change the key of the dictionary during iteration. But I was expecting a "RuntimeError: dictionary changed size during iteration" and nothing. I sometimes get a value, sometimes nothing ...

Python v2.7.10 on Osx

I modified my question because I just realize that this is the delete method that produces this result — Johaven, Jan 08 '16 at 18:19
I don't exhibit the same behaviour - if anything gets deleted I get a runtime error. Obviously changing from `iteritems()` to `items()` would fix the issue in Py2.X — AChampion, Jan 08 '16 at 18:25
I also tested your code here, changing the `v.update` because I don't know what kind of object you are using there, and I got a RuntimeError: dictionary changed size during iteration. I am using python 2.7 — Gabriel Ilharco, Jan 08 '16 at 18:27
yes update is an attribue of object v (setted to True or False) — Johaven, Jan 08 '16 at 18:29
http://stackoverflow.com/questions/5384914/how-to-delete-items-from-a-dictionary-while-iterating-over-it — mortymacs, Jan 08 '16 at 18:29

score 0 · Accepted Answer · answered Jan 08 '16 at 18:28

Breaks for me:

>>> mydict = {'a':1, 'b':10}
>>> for k, v in ((k, v) for k, v in mydict.iteritems() if v > 5):
...     print k
...     del mydict[k]
b
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
...

Obviously changing it to items() fixes this issue:

>>> mydict = {'a':1, 'b':10}
>>> for k, v in ((k, v) for k, v in mydict.items() if v > 5):
...     print k
...     del mydict[k]
b
>>> mydict
{'a': 1}

But a condition inside the loop is cleaner:

>>> mydict = {'a':1, 'b':10}
>>> for k, v in mydict.items():
...     if v > 5:
...         print k
...         del mydict[k]
b
>>> mydict
{'a': 1}

Or a dict comprehension:

>>> mydict = {'a':1, 'b':10}
>>> {k: v for k, v in mydict.iteritems() if not v > 5}
{'a': 1}

ok I found the culprit, inside a ThreadPool no error is raised ...sorry i forgot to mention it ... do I change the name of the topic? it's my first time — Johaven, Jan 08 '16 at 18:38

iteritems in generator and delete in Python2, no errors?

1 Answers1