OK, I assume your JSON looks like this (note the " around each value):

{"X":"value1", "Y":"value2", "Z":[{"A":"value3", "B":"value4"}]}

Then you can do this:

import json
j = '{"X":"value1", "Y":"value2", "Z":[{"A":"value3", "B":"value4"}]}'
k = json.loads(j)
assert k["Z"][0]["A"] == "value3"

Edit: Even simplejsoncan't decode your original input.

>>> import simplejson
>>> s1 = '{"X":value1,"Y":"value2","Z":[{"A":"value3","B":value4}]}'
>>> simplejson.loads(s1)
simplejson.decoder.JSONDecodeError: No JSON object could be decoded: line 1 column 0 (char 0)
>>> s2 = '{"X":"value1", "Y":"value2", "Z":[{"A":"value3", "B":"value4"}]}'
>>> print simplejson.loads(s2)["Z"][0]["A"]
value3

In this case json.load(data_file) should return a list of dictionaries. You can loop through each element of this list and access its dictionary keys and values. Specifically, what you are trying to do can be done as follows:

data = json.load(data_file)
for element in data: # data is a list
    print element['objectId'] # element is a dictionary

I used print because 'objectId' is a string. If it were a more complex structure you should have used pprint instead.

How about

data['cosponsors'][0]['thomas_id']

Since a list has numeric indices.