Why is flask's jsonify method slow?

Question

I'm writing an API in flask that returns json. Each flask function is of the form

from flask import jsonify
@app.route('/getdata')
def get_data():
    data = load_data_as_dict()
    return jsonify(data)

If I return a large amount of data, a call to this function takes around 1.7 seconds. However, if I do this:

from flask import Response
@app.route('/getdata')
def get_data():
    data = load_data_as_dict()
    data_as_str = json.dumps(data)
    return Response(response=data_as_str, status=200, mimetype="application/json"

...the function completes in around .05 seconds.

Can anyone tell me why jsonify is so much slower? Is there anything wrong with returning a raw Flask response instead?

Answer 1

My guess is: it has a lot to do with indentation and making a pretty json dump. Here's the method definition (I stripped the comments to save space, full code can be found here) :

def jsonify(*args, **kwargs):
    indent = None
    separators = (',', ':')

    if current_app.config['JSONIFY_PRETTYPRINT_REGULAR'] and not request.is_xhr:
        indent = 2
        separators = (', ', ': ')

    if args and kwargs:
        raise TypeError('jsonify() behavior undefined when passed both args and kwargs')
    elif len(args) == 1:  # single args are passed directly to dumps()
        data = args[0]
    else:
        data = args or kwargs

    return current_app.response_class(
        (dumps(data, indent=indent, separators=separators), '\n'),
        mimetype=current_app.config['JSONIFY_MIMETYPE']
    )

dumps wraps simplejson.dumps if the module is available, otherwise it uses json.dumps.