How to send data from server to client in Python using Flask?

Question

I'm making an application that sends data to a client, then the client prints the data.

I'm using Flask as a back-end framework that handles the server side, and another python script that generates random ID for the current client, the client checks every 4 seconds if new data comes in, if it gets data it should print it.

Back-end code

@app.route('/data/api/interact/<string:client_id>', methods=['GET', 'POST'])
@login_required
def interact(client_id):
    global data

    form = Interact()
    data = ''

    if form.is_submitted():
        get_data = form.ct.data

        if get_data == 'hello':
            data = 'Hi how are you?'

        return redirect(url_for('data_handler', client_id=client_id, data=form.ct.data))

    return render_template('interact.html', form=form, client_id=client_id)

@app.route('/data/api/interact/handler/', methods=['GET', 'POST'])
def data_handler():
    client_id = request.args.get('client_id')
    get_data = request.args.get('data')
    return json.dumps({'client_id': client_id, 'data': get_data})

Client script

handler_url = 'http://192.168.0.102:5000/data/api/interact/handler/'

class check_data(threading.Thread):
    def __init__(self, client_id):
        threading.Thread.__init__(self)
        self.event = threading.Event()
        self.client_id = client_id

    def run(self):
        global handler_url

        try:
            while not self.event.is_set():
                file = urllib2.urlopen(handler_url)
                xml = file.read()
                print xml
                file.close()
        except:
            pass

        self.event.wait(4)

def new_client():
        client_id = 'ClientId' + str(random.randrange(1, 500))
        return client_id

client_id = 'null'

while client_id == 'null':
    client_id = new_client()

    if 'null' not in client_id:
        break

print 'Client ID: ' + client_id

client = check_data(client_id)
client.start()

Everything works, but if I send data from the server to the client it prints:

{'data': '', 'client_id': null}

Answer 1

In this piece of code:

@app.route('/data/api/interact/handler', methods=['GET', 'POST'])
def data_handler():
    client_id = request.args.get('client_id')
    get_data = request.args.get('data')
    return json.dumps({'client_id': client_id, 'data': get_data})

You return JSON with client_id and data values, which you get from query parameters ( request.args ), but you don't send those parameters ( urllib2.urlopen(handler_url) ).

Answer 2

The client_id is not passed to the server, the server expects to get client_id and data as query parameters. Instead you are accessing the url without any parameters

file = urllib2.urlopen(handler_url) .

Passing query string parameters to GET request can be done with

url = handler_url + '?client_id' + self.client_id +'&data=YOURDATA_HERE'
file = urllib2.urlopen(url)

You can probably use urlencode to make this in a more elegant way.