I did not explain my questions clearly at beginning. Try to use str() and json.dumps() when converting json to string in python.
str()
json.dumps()
&
There are other differences. For instance, {'time': datetime.now()} cannot be serialized to JSON, but can be converted to string. You should use one of these tools depending on the purpose (i.e. will the result later be decoded).
{'time': datetime.now()}