How to implement server push in Flask framework?

As a follow-up to @peter-hoffmann’s answer, I’ve written a Flask extension specifically to handle server-sent events. It’s called Flask-SSE, and it’s available on PyPI. To install it, run:

$ pip install flask-sse

You can use it like this:

from flask import Flask
from flask_sse import sse

app = Flask(__name__)
app.config["REDIS_URL"] = "redis://localhost"
app.register_blueprint(sse, url_prefix='/stream')

@app.route('/send')
def send_message():
    sse.publish({"message": "Hello!"}, type='greeting')
    return "Message sent!"

And to connect to the event stream from Javascript, it works like this:

var source = new EventSource("{{ url_for('sse.stream') }}");
source.addEventListener('greeting', function(event) {
    var data = JSON.parse(event.data);
    // do what you want with this data
}, false);

Documentation is available on ReadTheDocs. Note that you’ll need a running Redis server to handle pub/sub.

Redis is overkill: use Server-Sent Events (SSE)

Late to the party (as usual), but IMHO using Redis may be overkill.

As long as you’re working in Python+Flask, consider using generator functions as described in this excellent article by Panisuan Joe Chasinga. The gist of it is:

In your client index.html

var targetContainer = document.getElementById("target_div");
var eventSource = new EventSource("/stream")
  eventSource.onmessage = function(e) {
  targetContainer.innerHTML = e.data;
};
...
<div id="target_div">Watch this space...</div>

In your Flask server:

def get_message():
    '''this could be any function that blocks until data is ready'''
    time.sleep(1.0)
    s = time.ctime(time.time())
    return s

@app.route('/')
def root():
    return render_template('index.html')

@app.route('/stream')
def stream():
    def eventStream():
        while True:
            # wait for source data to be available, then push it
            yield 'data: {}nn'.format(get_message())
    return Response(eventStream(), mimetype="text/event-stream")

As a committer of https://github.com/WolfgangFahl/pyFlaskBootstrap4 i ran into the same need and created a flask blueprint for Server Sent Events that has no dependency to redis.

This solutions builds on the other answers that have been given here in the past.

https://github.com/WolfgangFahl/pyFlaskBootstrap4/blob/main/fb4/sse_bp.py has the source code (see also sse_bp.py below).

There are unit tests at https://github.com/WolfgangFahl/pyFlaskBootstrap4/blob/main/tests/test_sse.py

The idea is that you can use different modes to create your SSE stream:

• by providing a function
• by providing a generator
• by using a PubSub helper class
• by using the PubSub helper class and use pydispatch at the same time.

As of 2021-02-12 this is alpha code which i want to share nevertheless. Please comment here or as issues in the project.

There is a demo at http://fb4demo.bitplan.com/events and a description of the example use e.g. for a progress bar or time display at: http://wiki.bitplan.com/index.php/PyFlaskBootstrap4#Server_Sent_Events

example client javascript/html code

<div id="event_div">Watch this space...</div>
<script>
    function fillContainerFromSSE(id,url) {
        var targetContainer = document.getElementById(id);
        var eventSource = new EventSource(url)
        eventSource.onmessage = function(e) {
            targetContainer.innerHTML = e.data;
        };
    };
    fillContainerFromSSE("event_div","/eventfeed");
</script>

example server side code

def getTimeEvent(self):
        '''
        get the next time stamp
        '''
        time.sleep(1.0)
        s=datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S')
        return s   

def eventFeed(self):
        '''
        create a Server Sent Event Feed
        '''
        sse=self.sseBluePrint
        # stream from the given function
        return sse.streamFunc(self.getTimeEvent)

sse_bp.py

'''
Created on 2021-02-06
@author: wf
'''
from flask import Blueprint, Response, request, abort,stream_with_context
from queue import Queue
from pydispatch import dispatcher
import logging

class SSE_BluePrint(object):
    '''
    a blueprint for server side events 
    '''
    def __init__(self,app,name:str,template_folder:str=None,debug=False,withContext=False):
        '''
        Constructor
        '''
        self.name=name
        self.debug=debug
        self.withContext=False
        if template_folder is not None:
            self.template_folder=template_folder
        else:
            self.template_folder='templates'    
        self.blueprint=Blueprint(name,__name__,template_folder=self.template_folder)
        self.app=app
        app.register_blueprint(self.blueprint)
        
        @self.app.route('/sse/<channel>')
        def subscribe(channel):
            def events():
                PubSub.subscribe(channel)
            self.stream(events)
                
    def streamSSE(self,ssegenerator): 
        '''
        stream the Server Sent Events for the given SSE generator
        '''  
        response=None
        if self.withContext:
            if request.headers.get('accept') == 'text/event-stream':
                response=Response(stream_with_context(ssegenerator), content_type='text/event-stream')
            else:
                response=abort(404)    
        else:
            response= Response(ssegenerator, content_type='text/event-stream')
        return response
        
    def streamGen(self,gen):
        '''
        stream the results of the given generator
        '''
        ssegen=self.generateSSE(gen)
        return self.streamSSE(ssegen)   
            
    def streamFunc(self,func,limit=-1):
        '''
        stream a generator based on the given function
        Args:
            func: the function to convert to a generator
            limit (int): optional limit of how often the generator should be applied - 1 for endless
        Returns:
            an SSE Response stream
        '''
        gen=self.generate(func,limit)
        return self.streamGen(gen)
                
    def generate(self,func,limit=-1):
        '''
        create a SSE generator from a given function
        Args:
            func: the function to convert to a generator
            limit (int): optional limit of how often the generator should be applied - 1 for endless
        Returns:
            a generator for the function
        '''   
        count=0
        while limit==-1 or count<limit:
            # wait for source data to be available, then push it
            count+=1
            result=func()
            yield result
        
    def generateSSE(self,gen):
        for result in gen:
            yield 'data: {}nn'.format(result)
            
    def enableDebug(self,debug:bool):
        '''
        set my debugging
        
        Args:
            debug(bool): True if debugging should be switched on
        '''
        self.debug=debug
        if self.debug:
            logging.basicConfig(level=logging.DEBUG, format='%(asctime)s.%(msecs)03d %(levelname)s:t%(message)s', datefmt='%Y-%m-%d %H:%M:%S')
            
    def publish(self, message:str, channel_str='sse', debug=False):
        """
        Publish data as a server-sent event.
        
        Args:
            message(str): the message to send
            channel(str): If you want to direct different events to different
                clients, you may specify a channel for this event to go to.
                Only clients listening to the same channel will receive this event.
                Defaults to "sse".
            debug(bool): if True  enable debugging
        """
        return PubSub.publish(channel=channel, message=message,debug=debug)

    def subscribe(self,channel,limit=-1,debug=False):
        def stream():
            for message in PubSub.subscribe(channel,limit,debug=debug):
                yield str(message)
                
        return self.streamGen(stream)
    
class PubSub:
    '''
    redis pubsub duck replacement
    '''
    pubSubByChannel={}
    
    def __init__(self,channel:str='sse',maxsize:int=15, debug=False,dispatch=False):
        '''
        Args:
            channel(string): the channel name
            maxsize(int): the maximum size of the queue
            debug(bool): whether debugging should be switched on
            dispatch(bool): if true use the pydispatch library - otherwise only a queue
        '''
        self.channel=channel
        self.queue=Queue(maxsize=maxsize)
        self.debug=debug
        self.receiveCount=0
        self.dispatch=False
        if dispatch:
            dispatcher.connect(self.receive,signal=channel,sender=dispatcher.Any)
        
    @staticmethod
    def reinit():
        '''
        reinitialize the pubSubByChannel dict
        '''
        PubSub.pubSubByChannel={}
        
    @staticmethod
    def forChannel(channel):    
        '''
        return a PubSub for the given channel
        
        Args:
            channel(str): the id of the channel
        Returns:
            PubSub: the PubSub for the given channel
        '''
        if channel in PubSub.pubSubByChannel:
            pubsub=PubSub.pubSubByChannel[channel]
        else:
            pubsub=PubSub(channel)
            PubSub.pubSubByChannel[channel]=pubsub
        return pubsub
    
    @staticmethod    
    def publish(channel:str,message:str,debug=False):
        '''
        publish a message via the given channel
        
        Args:
            channel(str): the id of the channel to use
            message(str): the message to publish/send
        Returns:
            PubSub: the pub sub for the channel
            
        '''
        pubsub=PubSub.forChannel(channel)
        pubsub.debug=debug
        pubsub.send(message)
        return pubsub
        
    @staticmethod    
    def subscribe(channel,limit=-1,debug=False): 
        '''
        subscribe to the given channel
        
        Args:
            channel(str): the id of the channel to use
            limit(int): limit the maximum amount of messages to be received        
            debug(bool): if True debugging info is printed
        '''  
        pubsub=PubSub.forChannel(channel)
        pubsub.debug=debug
        return pubsub.listen(limit)
    
    def send(self,message):
        '''
        send the given message
        '''
        sender=object();
        if self.dispatch:
            dispatcher.send(signal=self.channel,sender=sender,msg=message)
        else:
            self.receive(sender,message)
        
    def receive(self,sender,message):
        '''
        receive a message
        '''
        if sender is not None:
            self.receiveCount+=1;
            if self.debug:
                logging.debug("received %d:%s" % (self.receiveCount,message))
            self.queue.put(message)
        
    def listen(self,limit=-1):
        '''
        listen to my channel
        
        this is a generator for the queue content of received messages
        
        Args:
            limit(int): limit the maximum amount of messages to be received
        
        Return:
            generator: received messages to be yielded
        '''
        if limit>0 and self.receiveCount>limit:
            return
        yield self.queue.get()
    
    def unsubscribe(self):
        '''
        unsubscribe me
        '''
        if self.dispatch:
            dispatcher.disconnect(self.receive, signal=self.channel)
        pass