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?

11 Answers

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):

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]}

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]}

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:

For Python 3:

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

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]: 

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.

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)

For situations like this, i like to make a deep copy and loop through that copy while modifying the original dict.

If the lookup field is within a list, you can enumerate in the for loop of the list and then specify the position as index to access the field in the original dict.

dictc={“stName”:”asas”} keys=dictc.keys() for key in list(keys): dictc[key.upper()] =’New value’ print(str(dictc))

Python 3 does not allow deletion while iterating (using for loop above) dictionary. There are various alternatives to do; one simple way is the to change following line

for i in x.keys():


for i in list(x)

Leave a Reply

Your email address will not be published. Required fields are marked *