python RuntimeError: dictionary changed size during iteration

I have obj like this

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

It should be expand to

{ hello: 'world', foo: [{'bar': v1, 'name': v2}, {bar: v3}]}

I wrote code below, splite by '.', remove old key, append new key if contains '.', but it said RuntimeError: dictionary changed size during iteration

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

def expandField(obj, f, v):
    parts = f.split('.')
    if(len(parts) == 1):
        return
    del obj[f]
    for i in xrange(0, len(parts) - 1):
        f = parts[i]
        currobj = obj.get(f)
        if (currobj == None):
            nextf = parts[i + 1]
            currobj = obj[f] = re.match(r'd+', nextf) and [] or {}
        obj = currobj
    obj[len(parts) - 1] = v

for k, v in obj.iteritems():

RuntimeError: dictionary changed size during iteration

Answer

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.