Why is flask's jsonify method slow?

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.

jsonify() just wraps json.dumps(). However, depending upon the config of your Flask app and the Flask version that you're using, it may pass indent=2 and separators=(', ', ': ') to json.dumps. (See the docs on pretty-printing at https://docs.python.org/3/library/json.html if you're unfamiliar with these arguments).

Passing these arguments slows down json.dumps dramatically. Using the 181MB citylots.json file from https://github.com/zemirco/sf-city-lots-json as sample data, these pretty-printing arguments increase json.dumps()'s runtime from 7 seconds to 31 seconds on my MacBook Pro:

>>> import time >>> import json>>> citylots = json.load(open('citylots.json'))>>> start = time.time(); x = json.dumps(citylots); print(time.time() - start)7.165302753448486>>> x = None>>> start = time.time(); x = json.dumps(citylots, indent=2, separators=(', ', ': ')); print(time.time() - start)31.19125771522522

As of Flask 1.0, this costly pretty-printing will happen if either:

• You've explicitly set JSONIFY_PRETTYPRINT_REGULAR to True in your app's config (it's False by default), OR
• You're running your app in debug mode

(You can see these conditions in the Flask 1.0.2 code at https://github.com/pallets/flask/blob/1.0.2/flask/json/__init__.py#L309.)

If you are using Flask >=1.0 and have the (probably unusual) need to disable the pretty-printing even in debug mode, you can always implement your own jsonify by copying and pasting the built-in jsonify's definition and deleting all the pretty-printing logic:

from flask import current_appfrom json import dumpsdef jsonify(*args, **kwargs):    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) + '\n',        mimetype=current_app.config['JSONIFY_MIMETYPE']    )

If you're in a version Flask prior to 1.0, then pretty-printing instead happens if both:

• You haven't explicitly set JSONIFY_PRETTYPRINT_REGULAR to False in you app's config (it's True by default), AND
• The current request is not an XHR request

In those older versions, there is never any need to redefine jsonify to eliminate the pretty-printing, since you can just do:

app.config['JSONIFY_PRETTYPRINT_REGULAR'] = False

(Alternatively, if you're using a pre-1.0 version of Flask and only want to disable the pretty-printing in production, then there's no need to change your code; instead, just upgrade to the latest version of Flask.)

It took me a while to figure out, but Flask jsonify sets the sort_keys argument on the encoder and it seems it defaults to True.

Adding:

JSON_SORT_KEYS = False

To the configuration gave me a factor 7 speed up for larger JSON structures.