Python dictionary not updating as expected in a loop

I am trying to create a dictionary which summarizes another. I would like the summary_dict to only be updated when the key matches the "parent" value in the details_dict. What I have written does not seem to be accessing the summary_dict as I would expect. The print statements show that it keeps appending to the first iterable in the loop instead of grabbing the right value from the summary_dict.

detail_dict = {'a123': {"data": [1, 2, 3, 4], "parent": "a"},
               'a124': {"data": [1, 2, 3, 4, 5], "parent": "a"},
               'b123': {"data": [1, 2, 3], "parent": "b"},
               'b124': {"data": [1], "parent": "b"}}

summary_dict = dict.fromkeys(["a", "b"], {"data": [],
                                          "data_len": []})

for k, v in detail_dict.iteritems():
    summary_dict[v['parent']]["data"].append(v["data"])
    summary_dict[v['parent']]["data_len"].append(len(v["data"]))
    print "\nMy value is "
    print v
    print "\nMy summary dictionary now looks like:"
    print summary_dict[v['parent']]

The resultant dictionary I would like is:

{"a": {"data": [[1, 2, 3, 4], [1, 2, 3, 4, 5]],
       "data_len": [4, 5]},
 "b": {"data": [[1, 2, 3], [1]],
       "data_len": [3, 1]}}

you're passing a mutable parameter to from_keys so the reference is copied between keys.

Create your dict like this instead to create one separate ref for each key:

summary_dict = {x : {"data": [],"data_len": []} for x in ["a","b"]}

note that as the order of the dict is not guaranteed you get something like:

{'a': {'data_len': [5, 4], 'data': [[1, 2, 3, 4, 5], [1, 2, 3, 4]]}, 'b': {'data_len': [3, 1], 'data': [[1, 2, 3], [1]]}}

you could use

for k, v in sorted(detail_dict.items()):

to sort the items so order in the list is deterministic.