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

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Question :

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?

Answer #1:

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):
Answered By: Mark Byers

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.

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]}
Answered By: Alon Elharar

Answer #3:

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]}

For this in Python 3

>>> d
{'a': [1], 'c': [], 'b': [1, 2], 'd': []}
>>> d = { k : v for k,v in d.items() if v}
>>> d
{'a': [1], 'b': [1, 2]}
Answered By: Maria Zverina

Answer #4:

I would try to avoid inserting empty lists in the first place, but, would generally use:

d = {k: v for k,v in d.iteritems() if v} # re-bind to non-empty

If prior to 2.7:

d = dict( (k, v) for k,v in d.iteritems() if v )

or just:

empty_key_vals = list(k for k in k,v in d.iteritems() if v)
for k in empty_key_vals:
Answered By: Jon Clements

Answer #5:

For Python 3:

{k:v for k,v in d.items() if v}
Answered By: ucyo

Answer #6:

You cannot iterate through a dictionary while its changing during for loop. Make a casting to list and iterate over that list, it works for me.

    for key in list(d):
        if not d[key]: 
Answered By: Alvaro Romero Diaz

Answer #7:

This worked for me:

dict = {1: 'a', 2: '', 3: 'b', 4: '', 5: '', 6: 'c'}
for key, value in list(dict.items()):
    if (value == ''):
        del dict[key]
# dict = {1: 'a', 3: 'b', 6: 'c'}  

Casting the dictionary items to list creates a list of its items, so you can iterate over it and avoid the RuntimeError.

Answered By: singrium

Answer #8:

The reason for the runtime error is that you cannot iterate through a data structure while its structure is changing during iteration.

One way to achieve what you are looking for is to use list to append the keys you want to remove and then use pop function on dictionary to remove the identified key while iterating through the list.

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

for i in d:
        if not d[i]:

for x in pop_list:
print (d)
Answered By: Rohit

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