python: read json and loop dictionary

That seems generally fine.

There's no need to first read the file, then use loads. You can just use load directly.

output_json = json.load(open('/tmp/output.json'))

Using i and k isn't correct for this. They should generally be used only for an integer loop counter. In this case they're keys, so something more appropriate would be better. Perhaps rename i as container and k as stream? Something that communicate more information will be easier to read and maintain.

You can use output_json.iteritems() to iterate over both the key and the value at the same time.

for majorkey, subdict in output_json.iteritems():
    print majorkey
    for subkey, value in subdict.iteritems():
            print subkey, value

Note that, when using Python 3, you will need to use items() instead of iteritems(), as it has been renamed.