How to avoid “RuntimeError: dictionary changed size during iteration” error?

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I have checked all of the other questions with the same error yet found no helpful solution =/

I have a dictionary of lists:

d = {'a': [1], 'b': [1, 2], 'c': [], 'd':[]}

in which some of the values are empty. At the end of creating these lists, I want to remove these empty lists before returning my dictionary. Current I am attempting to do this as follows:

for i in d:
    if not d[i]:
        d.pop(i)

however, this is giving me the runtime error. I am aware that you cannot add/remove elements in a dictionary while iterating through it…what would be a way around this then?


See Modifying a Python dict while iterating over it for citations that this can cause problems, and why.


Solution

In Python 3.x and 2.x you can use use list to force a copy of the keys to be made:

for i in list(d):

In Python 2.x calling keys made a copy of the keys that you could iterate over while modifying the dict:

for i in d.keys():

But note that in Python 3.x this second method doesn’t help with your error because keys returns an a view object instead of copynig the keys into a list.

Source: StackOverflow.com

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