Jessica McKellar added the comment:
I want to state explicitly what the error is for some new contributors who
might pick this up at a sprint this weekend:
The issue is that you can't change a dictionary while iterating over it:
>>> d = {"a": "b"}
>>> for elt in d.keys():
... del d[elt]
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration
In Python 2, d.keys() produced a copy of the list of keys, and we delete items
from the dictionary while iterating over that copy, which is safe. In Python 3,
d.keys() produces a view object* instead, and mutating the dictionary while
iterating over it is unsafe and raises an error.
The patch makes a copy of the keys before iterating over it so that it is safe
in Python 3.
* https://docs.python.org/3/library/stdtypes.html#dict-views
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nosy: +jesstess
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<http://bugs.python.org/issue21463>
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