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

user_1530318 0
Viewed 426695

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.

list dictionary python loops
12 Answers

This worked for me:

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

user_81281900

to avoid "dictionary changed size during iteration error".

for example : "when you try to delete some key" ,

just use 'list' with '.items()' , and here is a simple example :

my_dict = {
    'k1':1,
    'k2':2,
    'k3':3,
    'k4':4
 
    }
    
print(my_dict)

for key, val in list(my_dict.items()):
    if val == 2 or val == 4:
        my_dict.pop(key)

print(my_dict)

+++ output :

{'k1': 1, 'k2': 2, 'k3': 3, 'k4': 4}

{'k1': 1, 'k3': 3}

+++

this is just example and change it based on your case/requirements, i hope this helpful.

user_126620560

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]: 
            d.pop(key)
user_106468020

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():

With 

for i in list(x)
user_111072440

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]:
                pop_list.append(i)

for x in pop_list:
        d.pop(x)
print (d)
user_104467610

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.

user_15405690 
dictc={"stName":"asas"}
keys=dictc.keys()
for key in list(keys):
    dictc[key.upper()] ='New value'
print(str(dictc))
user_4866310
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