python - Why should json.loads be preferred to ast.literal_eval for parsing JSON?

Question

I have a dictionary that is stored in a db field as a string. I am trying to parse it into a dict, but json.loads gives me an error.

Why does json.loads fail on this and ast.literal_eval works? Is one preferable over the other?

>>> c.iframe_data
u"{u'person': u'Annabelle!', u'csrfmiddlewaretoken': u'wTE9RZGvjCh9RCL00pLloxOYZItQ98JN'}"

# json fails
>>> json.loads(c.iframe_data)
Traceback (most recent call last):
ValueError: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)

# ast.literal_eval works
>>> ast.literal_eval(c.iframe_data)
{u'person': u'Annabelle!', u'csrfmiddlewaretoken': u'wTE9RZGvjCh9RCL00pLloxOYZItQ98JN'}

Answer 1

json.loads failed because your c.iframe_data value is not a valid JSON document. In valid json document string are quoted in double quote and there isn't anything like u for converting strings to unicode.

Using json.loads(c.iframe_data) means deserialize the JSON document in c.iframe_data

ast.literal_eval is used whenever you need eval to evaluate input expression. If you have Python expressions as an input that you want to evaluate.

Is one preferable over the other?

It depends on the data. See this answer for more context.