Using your code, this is how I would do it. I know an answer was chosen, just giving additional options.
data = json.loads('{"lat":444, "lon":555}')
ret = ''
for j in data:
ret = ret+" "+data[j]
return ret
When you use for in this manor you get the key of the object, not the value, so you can get the value, by using the key as an index.