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


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]:

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?

5/11/2016 7:13:44 PM

Accepted Answer

In Python 2.x calling keys makes a copy of the key that you can iterate over while modifying the dict:

for i in d.keys():

Note that this doesn't work in Python 3.x because keys returns an iterator instead of a list.

Another way is to use list to force a copy of the keys to be made. This one also works in Python 3.x:

for i in list(d):
5/11/2016 7:16:43 PM

Just use dictionary comprehension to copy the relevant items into a new dict

>>> d
{'a': [1], 'c': [], 'b': [1, 2], 'd': []}
>>> d = { k : v for k,v in d.iteritems() if v}
>>> d
{'a': [1], 'b': [1, 2]}

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