python RuntimeError: dictionary changed size during iteration

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.

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!

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

def expand(obj):
    keys = list(obj.keys())  # freeze keys iterator into a list
    for k in keys:
        expandField(obj, k, v)

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

Edited as per comments, thank you !