python RuntimeError: dictionary changed size during iteration

Question

I have obj like this

{hello: \'world\', \"foo.0.bar\": v1, \"foo.0.name\": v2, \"foo.1.bar\": v3}

It should be expand to

{         


        

Answer 1

Rewriting this part

def expand(obj):
    for k in obj.keys():
        expandField(obj, k, v)

to the following

def expand(obj):
    keys = obj.keys()
    for k in keys:
        if k in obj:
            expandField(obj, k, v)

shall make it work.

Answer 2

For those experiencing

RuntimeError: dictionary changed size during iteration

also make sure you're not iterating through a defaultdict when trying to access a non-existent key! I caught myself doing that inside the for loop, which caused the defaultdict to create a default value for this key, causing the aforementioned error.

The solution is to convert your defaultdict to dict before looping through it, i.e.

d = defaultdict(int)
d_new = dict(d)

or make sure you're not adding/removing any keys while iterating through it.

Answer 3

I had a similar issue with wanting to change the dictionary's structure (remove/add) dicts within other dicts.

For my situation I created a deepcopy of the dict. With a deepcopy of my dict, I was able to iterate through and remove keys as needed.Deepcopy - PythonDoc

A deep copy constructs a new compound object and then, recursively, inserts copies into it of the objects found in the original.

Hope this helps!

Answer 4

You might want to copy your keys in a list and iterate over your dict using the latter, eg:

def expand(obj):
    keys = obj.keys()
    for k in keys:
        expandField(obj, k, v)

I let you analyse if the resulting behavior suits your expected results.

Answer 5

Like the message says: you changed the number of entries in obj inside of expandField() while in the middle of looping over this entries in expand.

You might try instead creating a new dictionary of the form you wish, or somehow recording the changes you want to make, and then making them AFTER the loop is done.