Just do a simple .keys()

>>> dct = {
...     "1": "a", 
...     "3": "b", 
...     "8": {
...         "12": "c", 
...         "25": "d"
...     }
... }
>>> 
>>> dct.keys()
['1', '8', '3']
>>> for key in dct.keys(): print key
...
1
8
3
>>>

If you need a sorted list:

keylist = dct.keys() # this is of type `dict_key`, NOT a `list`
keylist.sort()

And if you want them as simple list, do this:

list(dct_instance.keys())

If you want to iterate over both keys and values of the dictionary, do this:

for key, value in data.items():
    print(key, value)

Function which return only keys which aren't containing dictionary as their value.

jsonData = {
    "a": 1,
    "b": 2,
    "c": [{
        "d": 4,
        "e": 5,
        "f": {
            "g": 6
        }
    }]
}


def get_simple_keys(data):
    result = []
    for key in data.keys():
        if type(data[key]) != dict:
            result.append(key)
        else:
            result += get_simple_keys(data[key])
    return result

print get_simple_keys(jsonData['c'][0])

To avoid using recursion change line result += get_simple_keys(data[key])to result += data[key].keys()

First off when the data is loaded from json it creates a dict object, so all the methods described in the documentation are usable including keys:

print(jsonString['result']['cats'].keys())

you can see a list of all the possible methods with dir(dict) or dir(jsonString) and get help on any of them with the built in function:

>>> help(dict.keys)
Help on method_descriptor:

keys(...)
    D.keys() -> a set-like object providing a view on D's keys