Flask jsonify returns weird array?

Question

I'm trying to create a simple API using Flask. I now want to return a list of dicts as follows:

print results  # prints out [{'date': '2014-09-25 19:00:00', 'title': u'Some Title'}]
response = make_response(jsonify(results))
response.headers['Content-Type'] = 'application/json'
return response

But when I go the the url in the browser, I get the following:

{
  "date": "title"
}

Does anybody know what I'm doing wrong here? All tips are welcome!

Answer 1

This is only an issue for Flask versions before 0.11; if you still see this problem the best thing to do is to upgrade to a later version, as 0.10 is quite ancient by now.

---

For Flask up to version 0.10, jsonify() will only accept dictionaries. If you give it a list, it'll turn the object into a dictionary, with dict(argument). See the Flask.jsonify() documentation:

Creates a Response with the JSON representation of the given arguments with an application/json mimetype. The arguments to this function are the same as to the dict constructor.

(Emphasis mine)

In your case you have a list with one element, and that element, when iterated over, has 2 values. Those two values then become the key and value of the output dictionary:

>>> results = [{'date': '2014-09-25 19:00:00', 'title': u'Some Title'}]
>>> dict(results)
{'date': 'title'}

That's because the dict() constructor either takes another dictionary, keyword arguments or an iterable of (key, value) pairs.

The solution is to not pass in a list, but give it, at the very least, a key:

response = jsonify(results=results)

jsonify() already returns a response object, there is no need to call make_response() on it. The above produces a JSON object with a 'results' key and your list as the value.

jsonify() only takes a dictionary for security reasons. Quoting the documentation again:

For security reasons only objects are supported toplevel. For more information about this, have a look at JSON Security.

If you really want to bypass this, you'll have to create your own response:

from Flask import json

response = make_response(json.dumps(results), mimetype='application/json')

Answer 2

This should no longer be an issue now that Flask's jsonify() method serializes top-level arrays (as of this commit).

You'll need to be on Flask >= 0.11.

For convenience, you can either pass in a Python list: jsonify([1,2,3]) Or pass in a series of args: jsonify(1,2,3)

Both will be converted to: [1,2,3]

Details here: http://flask.pocoo.org/docs/dev/api/#flask.json.jsonify