Flask's jsonify in 2020

Thu, May 14, 2020 2-minute read

Flask jsonify, do I still need it?

For a long time, returning JSON in flask required using the flask.jsonify API. However, since this PR Flask will by default call jsonify under the hood on any dictionary it receives on a make_response call.

How does it work

When you need to return json in a response, you can simply use a plain python dictionary instead of the jsonify API.

# plain dictionary
    def testplain():                                                                          
        test = 'testing'                                                                 
        name = 'roger'                                                                   
        age =  30                                                                        
        _list = [1,2,3]                                                                    
        return {                                                                         
                "development": test,                                                     
                "name": name,                                                            
                "age": age,                                                              
                "list": _list                                                              

The above code will return the following:

# response
HTTP/1.0 200 OK
Content-Length: 105
Content-Type: application/json
Date: Thu, 14 May 2020 21:20:06 GMT
Server: Werkzeug/1.0.1 Python/3.8.2

    "age": 30,
    "development": "testing",
    "list": [
    "name": "roger"

And the same code but with jsonify (for comparison) will return the exact same thing.

   def testjsonify():                                                                   
        test = 'testing'                                                                 
        name = 'roger'                                                                   
        age =  30                                                                        
        lst = [1,2,3]                                                                    
        return jsonify({                                                                 
                 "development": test,                                                     
                 "name": name,                                                            
                 "age": age,                                                              
                 "list": lst                                                              

How does it work

As stated above, Flask now does an isinstance check during the make_response API call. In the conditional, Flask checks if the body is of type dict and if so calls jsonify. To see this in action refer to the previously mentioned line in the core Flask app.py.

So, while we no longer need to explicitly call jsonify it is still very much being used by the application itself.

Whats the catch?

There is one small gotcha, as seen from pgjones pull request.

This doesn’t support returning anything other than an associate array at the top level in the JSON response. I’m ok with this as in practice APIs are only extensible if the top level is an associate array.

An example of when you will still need to user jsonify is when the response is not of type dict. As an example, doing a list comprehension and returning that directly will be required to be wrapped in jsonify.

    def users_api():
        users = get_all_users()
        return jsonify([user.to_json() for user in users])


While its not a complete replacement for flask.jsonify, it will probably reduce its explicit usage by a huge proportion. Personally, I removed jsonify from most of my applications and now just drop in plain dictionaries.

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