Flask not streaming JSON response

Question

I have below flask application that should stream JSON response

Directory structure:

server - 
        api.py
        resources -
                   stream.py

api.py

from flask import Flask
from flask_restful import Api
from resources.stream import Stream
from flask_compress import Compress

compress = Compress()
app = Flask(__name__)
compress.init_app(app)
api = Api(app)
api.add_resource(Stream, '/stream')
if __name__ == '__main__':
    app.run(debug=False,host='0.0.0.0')

stream.py in resources directory

from flask import Response, request, stream_with_context
from flask_restful import Resource

class Stream(Resource):
    def get(self):
        def generator():
            yield '{"data": ['
            #creating large number of entries for data array with yield
            yield ']}'
        resp = Response(stream_with_context(generator()), status=200, content_type='application/json')
        return resp

I started the flask app with python3.9 api.py

I am able to get response when I hit url http://127.0.0.1:5000/stream but in network tab I can see some issues:

If the response was streamed it should not have content-length
The streamed response should not be in Waiting for server response state, somehow its waiting to finish whole response and then starts downloading.

And below is the output that I am trying to achieve. A streamed output which would start Content Downloading and not be stuck in Waiting for server response

maybe it needs some special header to inform that you stearm data. — furas
– furas, Commented Aug 8, 2022 at 10:02
Tried your code with flask 1.1.2 and werkzeug 1.0.1 (my current stack) worked as expected. Maybe the problem is elsewhere, did you have a look at this : stackoverflow.com/a/23648948/2307934 — edg
– edg, Commented Aug 9, 2022 at 8:37
Thanks for the tip regarding Content-Length, that helped me fix my issue with this. — Matthew Moisen
– Matthew Moisen, Commented Jun 17, 2023 at 16:09

Pritam · Accepted Answer · 2022-09-01 08:56:56Z

0

I found the solution for the problem I faced. I was using flask_compress which was causing this issue. Removing it would stream the response as expected. Thank you for the help though.

answered Sep 1, 2022 at 8:56

Pritam

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Matthew Moisen · Accepted Answer · 2023-06-17 16:09:17Z

0

For me, I had some code in my after_request function that called response.data, essentially:

def after_request(response):
    logging.info(response.data)

Apparently, doing this will convert the stream back into a normal response, for example setting Content-Length and removing the Transfer-Encoding: chunked.

I changed this to the following which removed this issue.

def after_request(response):
    if response.is_streamed:
        logging.info('<STREAMED>')
    else:
        logging.info(response.data)

answered Jun 17, 2023 at 16:09

Matthew Moisen

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