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

Question

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?

Answer 1

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.

Answer 2

You only need to use "copy":

On that's way you iterate over the original dictionary fields and on the fly can change the desired dict (d dict). It's work on each python version, so it's more clear.

(BTW - Generally to iterate over copy of your data structure, instead of using ".copy" for "dict" or slicing "[:]" for "list", you can use import copy -> copy.copy (for shallow copy which equivalent to "copy" that supported by "dict" or slicing "[:]" that supported by "list") or copy.deepcopy on your data structure).

In [1]: d = {'a': [1], 'b': [1, 2], 'c': [], 'd':[]}  In [2]: for i in d.copy():    ...:     if not d[i]:    ...:         d.pop(i)    ...:           In [3]: d Out[3]: {'a': [1], 'b': [1, 2]}