I'm trying to do long polling with JQuery and Python under the Flask Framework.
Having done long polling before in PHP, I've tried to go about it in the same way:
A script/function that has a while(true) loop, checking for changes periodically eg.every 0,5 seconds in the database, and returns some data when a change occurs.
So in my ini.py I've created an app.route to /poll for JQuery to call. JQuery gives it some information about the client's current state, and the poll() function compares this with what's currently in the database. The loop is ended and returns information when a change is observed.
Here's the python code:
#app.route('/poll')
def poll():
client_state = request.args.get("state")
#remove html encoding + whitesapce from client state
html_parser = HTMLParser.HTMLParser()
client_state = html_parser.unescape(client_state)
client_state = "".join(client_state.split())
#poll the database
while True:
time.sleep(0.5)
data = get_data()
json_state = to_json(data)
json_state = "".join(data) #remove whitespace
if json_state != client_state:
return "CHANGE"
The problem is that, when the code above starts polling, the server appears to be overloaded and other Ajax calls, and other requests like loading a "loading" image to the html using JQuery are unresponsive and timeout.
For completion's sake I've included the JQuery here:
function poll() {
queryString = "state="+JSON.stringify(currentState);
$.ajax({
url:"/poll",
data: queryString,
timeout: 60000,
success: function(data) {
console.log(data);
if(currentState == null) {
currentState = JSON.parse(data);
}
else {
console.log("A change has occurred");
}
poll();
},
error: function(jqXHR, textStatus, errorThrown) {
console.log(jqXHR.status + "," + textStatus + ", " + errorThrown);
poll();
}
});
}
Does this need to multi-threaded or something? Or does anyone have any idea why I'm experiencing this behavior?
Thanks in advance!! :)
Just as the link #Robᵩ mentioned, you flask app is just overload. That's because a flask app is in single threading mode by default when running with app.run(), so it can only serve one request per time.
You can start multi threading with:
if __name__ == '__main__':
app.run(threaded=True)
Or using a WSGI server like gunicorn or uwsgi to serve flask with multi processing:
gunicorn -w 4 myapp:app
Hopes you're enjoying with Python and Flask!
Related
Overview
When creating a post request from my website to my Python server running CherryPy, I receive the error Access to XMLHttpRequest has been blocked by CORS policy: Request header field content-type is not allowed by Access-Control-Allow-Headers in preflight response. . I was able to get away with the problem temporarily with one of the "CORS Everywhere" browser extensions, but
Due to recent updates, the extensions have not yet been updated to be working again.
The website involved needs to eventually be used by many in my local complex without the browser extension, so once the extensions get updated, it does not really matter one way or another, as I cannot rely on these extensions, and force everyone to use them (when there is obviously a fix that would make an extension not necessary).
I figure that perhaps the solutions are outdated, but am not sure.
Here is the relevant code:
On the server side (CherryPy/Python):
The CherryPy Python function being called, from the website post request
#cherrypy.expose
#cherrypy.tools.json_in()
def add_meeting(self):
data = None
id = None
start_time = None
end_time = None
title = None
userlist = None
result = {"operation": "request", "result": "success"}
if cherrypy.request.method == "POST":
data = cherrypy.request.json
id = data["id"]
start_time = data["start_time"]
end_time = data["end_time"]
title = data["title"]
userlist = data["userlist"]
# Rest of relevant code in function is left out, to take up less
# space and not post irrelevant code. That being said, I am
# positive the logic is correct, as it originally ran smoothly
# with a "Cors Everywhere" Browser Extension.
return result
Here is the area where I set up and run CherryPy
def main():
# Create the configuration file parser object and start the CherryPy server
config = ConfigParser.ConfigParser()
config.read(CONFIG_FILE)
port = config.getint('Meta', 'port')
host = config.get('Meta', 'host')
cherrypy.config.update({'server.socket_port': port,
'server.socket_host': host,
'tools.CORS.on': True})
cherrypy.quickstart(Coordinator(config))
main()
Here is the config file mentioned in the code above (CONFIG_FILE)
[Meta]
host = 0.0.0.0
port = 3000
# Rest is left out, as it is irrelevant with problem
The solutions I have tried implementing
The inclusion of the following function above the main function:
def CORS():
cherrypy.response.headers["Access-Control-Allow-Origin"] = "*"
with cherrypy.tools.CORS = cherrypy.Tool('before_handler', CORS)
2. Adding " 'cors.expose.on': True " to cherrypy.config.update above
3. Using this cherrypy-cors Python library I found online: https://pypi.org/project/cherrypy-cors/
4. The inclusion of headers in the config.update portion of the Python file
5. Adding "#cherrypy.tools.accept(media='application/json')" before "def add_meeting"
Conclusion
I've tried the solutions above together, separately, some with and without the others, and I am still stuck. Maybe some of these solutions are partially correct, and there is something extra needed with my code. I am not sure; I just cannot get it working. I do not have much experience with web development before this, so maybe (and hopefully) the solution is extremely simple. I know the code works, I just cannot get it running without a working "Cors Everywhere" browser extension for every user.
As for the versions I am running: I am using CherryPy 14.2.0 and Python 2.7.6
Any help would mean the absolute world to me, thank you.
So first, you need to set pre-flight headers when processing OPTIONS request, you can list allowed methods there.
Then, you also need to enable the cors.expose tool.
There's some usage hints in the docstring of cherrypy-cors. For example, when using a MethodDispatcher, you could just decorate an OPTIONS handler method with #cherrypy_cors.tools.preflight() instead of doing this in every HTTP handler.
Here's a simple traversal example (without a method dispatcher). To test it, visit http://127.0.0.1/ and it will make requests against http://localhost:3333/add_meeting which is a different Origin in terms of CORS ('localhost' != '127.0.0.1').
"""Example of CORS setup using cherrypy-cors library."""
import cherrypy
import cherrypy_cors
# Python 2 compat: make all classes new-style by default
__metaclass__ = type # pylint: disable=invalid-name
class WebRoot:
"""Root node for HTTP handlers."""
#cherrypy.expose
def index(self): # pylint: disable=no-self-use
"""Render a web page handling request against ``/``.
Contains client JS snippet which will query the API endpoint.
It will be executed by the browser while loading the page.
"""
return """<html>
<script type="text/javascript">
async function addMeeting() {
/*
* Example coroutine for querying /add_meeing
* HTTP endpoint. It uses localhost as in the URL.
* For testing CORS, make sure to visit
* http://127.0.0.1/ which is a different origin
* from browser's perspective.
* /
const request_payload = {
some: 'data',
listed: ['h', 'er', 'e'],
}
try {
const resp = await fetch(
'http://localhost:3333/add_meeting',
{
method: 'POST',
mode: 'cors', // Required for customizing HTTP request headers
credentials: 'same-origin',
headers: {
'Content-Type': 'application/json; charset=UTF-8', // Required for ``cherrypy.tools.json_in`` to identify JSON payload and parse it automatically
},
body: JSON.stringify(request_payload),
},
)
const json_resp = await resp.json()
console.log(json_resp) // Will print: {"method": "POST", "payload": {"listed": ["h", "er", "e"], "some": "data"}}
} catch (e) {
console.warn('Exception: ' + e)
}
}
async function main() {
await addMeeting()
}
main() // Entry point
</script>
</html>""" # noqa: E501
#cherrypy.expose
#cherrypy.tools.json_in() # turn HTTP payload into an object; also checking the Content-Type header
#cherrypy.tools.json_out() # turn ``return``ed Python object into a JSON string; also setting corresponding Content-Type
def add_meeting(self):
"""Handle HTTP requests against ``/add_meeting`` URI."""
if cherrypy.request.method == 'OPTIONS':
# This is a request that browser sends in CORS prior to
# sending a real request.
# Set up extra headers for a pre-flight OPTIONS request.
cherrypy_cors.preflight(allowed_methods=['GET', 'POST'])
if cherrypy.request.method == 'POST':
return {'method': 'POST', 'payload': cherrypy.request.json}
return {'method': 'non-POST'}
def main():
"""Set up and run the web app.
Initializes CORS tools.
Sets up web server socket.
Enables the CORS tool.
"""
cherrypy_cors.install()
cherrypy.config.update({
'server.socket_host': '127.0.0.1',
'server.socket_port': 3333,
'cors.expose.on': True,
})
cherrypy.quickstart(WebRoot())
__name__ == '__main__' and main() # pylint: disable=expression-not-assigned
UPDATE
For me the Problem got fixed as soon as I was putting "encoding: URLEncoding(destination: .queryString)" in my request. Maybe this helps somebody else. link
I struggled the whole day to find the problem in my Alamofire PUT Request or the Flask Restful API. Request like GET, DELETE and POST are working fine with Alamofire, except the PUT Request.
When I'm using PUT Requests in combination with Postman and Flask-Restful everything is also working fine. But as soon as I'm trying to achieve the same Result with Alamofire, I'm not getting any parameters in Flask. I tried to illustrate this in the code examples.
So in short my example illustrates the following:
DELETE Request(Same with GET and POST)
Postman: success
Alamofire: success
PUT Request
Postman: success
Alamofire: failure (parameter dictionary empty in Flask-Restful)
Here is my Python Code [API Server]:
from flask import Flask, request, jsonify
from flask_restful import Resource, Api, reqparse
app = Flask(__name__)
api = Api(app)
class Stackoverflow(Resource):
def delete(self):
print(request.args)
if request.args.get('test-key') is None:
return jsonify({"message": "failure"})
else:
return jsonify({"message": "success"})
def put(self):
print(request.args)
if request.args.get('test-key') is None:
return jsonify({"message": "failure"})
else:
return jsonify({"message": "success"})
api.add_resource(Stackoverflow, '/stackoverflow')
if __name__ == '__main__':
app.run(debug=True, host='0.0.0.0')
If I'm using Postman, I get this result (like expected):
Result in Postman
But now I'm trying to do the same with Alamofire in Swift. Same Server, nothing changed.
SWIFT demo Code [IOS APP]:
import UIKit
import Alamofire
import SwiftyJSON
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view
simplePUTRequest()
simpleDELETERequest()
}
func simplePUTRequest(){
AF.request("http://localhost:5000/stackoverflow", method: .put, parameters: ["test-key":"testvalue"])
.validate(statusCode: 200..<300)
.responseJSON { response in
if let data = response.data {
print("Result PUT Request:")
print(String(data: data, encoding: .utf8)!)
//print(utf8Text)
}else{
}
}
}
func simpleDELETERequest(){
AF.request("http://localhost:5000/stackoverflow", method: .delete, parameters: ["test-key":"testvalue"])
.validate(statusCode: 200..<300)
.responseJSON { response in
if let data = response.data {
print("Result DELETE Request:")
print(String(data: data, encoding: .utf8)!)
//print(utf8Text)
}else{
}
}
}
Xcode Console:
Result PUT Request:
{
"message": "failure"
}
Result DELETE Request:
{
"message": "success"
}
python Console (both Alamofire Requests):
ImmutableMultiDict([])
127.0.0.1 - - [15/Jun/2019 21:17:31] "PUT /stackoverflow HTTP/1.1" 200 -
ImmutableMultiDict([('test-key', 'testvalue')])
127.0.0.1 - - [15/Jun/2019 21:17:31] "DELETE /stackoverflow?test-key=testvalue HTTP/1.1" 200 -
As you can see, I'm getting the success message only while using the DELETE method.
Till now I tried using different encodings like URLEncoding.httpbody and URLEncoding.default, but nothing really helped.
For me it seems like it's a Alamofire/Swift Problem, because in Postman the same request method is working fine.
I would really appreciate your help, because I'm stuck and don't know anything further to do. I hope I didn't misunderstood something essential.
Thank you in advance!
I am currently using the same version AlamoFire, and when I use the PUT method, I use it as follows:
let request = AF.request(url, method: .put, parameters: ["uid": uid],
encoding: JSONEncoding.default, headers: headers)
request.responseJSON(completionHandler: { response in
guard response.error == nil else {
//Handle error
}
if let json = response.value as? [String: Any]
// Handle result.
}
The only difference to your post is that I used the encoding option. You can try to put the option and see what happens.
It looks like your server is expecting your PUT parameters to be URL form encoded into the URL. You may be hitting the version of the request method that uses JSON encoding by default, so adding encoder: URLEncodedFormParameterEncoder.default at the end of your request call should fix that. A future release will make that the default, as it's safe across all request types.
If that's not the issue, I suggest you investigate more closely to see what the differences between the requests may be. Since you control the server you should have easy access to the traffic.
I was playing around the rabbitmq HTTP API and came across a weird scenario. When I look at my queues through the web interface, the status of both of them shows as IDLE. .
However when I use the HTTP API, the return for both the queue shows as 'running'. The code im using is below:
import requests
import json
uri = 'http://localhost:15672/api/queues'
r = requests.get(uri, auth=("guest","guest"))
parsed = json.loads(r.content)
#print json.dumps(parsed, indent=4)
for i in parsed:
print '{:<20} : {}'.format(i.get('name'), i.get('state'))
Output:
test queue : running
test2 : running
Can someone explain this behaviour to me?
Check the Management_console source code here: https://github.com/rabbitmq/rabbitmq-management/blob/master/priv/www/js/formatters.js#L479
function fmt_object_state(obj) {
if (obj.state == undefined) return '';
var colour = 'green';
var text = obj.state;
var explanation;
if (obj.idle_since !== undefined) {
colour = 'grey';
explanation = 'Idle since ' + obj.idle_since;
text = 'idle';
}
The console shows "idle" if the field idle_since is not null.
If there is "traffic" in your queue you will have a json like that:
"policy":"",
"exclusive_consumer_tag":"",
"consumers":0,
"consumer_utilisation":"",
"memory":176456,
"recoverable_slaves":"",
"state":"running",
if the queue is in idle (without traffic) you will have a json like that:
"idle_since":"2015-06-25 10:15:07",
"consumer_utilisation":"",
"policy":"",
"exclusive_consumer_tag":"",
"consumers":0,
"recoverable_slaves":"",
"state":"running",
As you can see the field "idle_since" is not null.
In both cases the queue is always in running state.
In conclusion it is just a web-view formatting.
How to use python as a backend for an Android App that is built using C#? The Python Backend is written using the Flask framework. The Android app is built using xamarin.
No matter what type of technology your server or the client use if they can communicate with each other using some sort of standard "protocol".
There are many ways to communicate both sides (client and server) like sockets, xml, json, etc. They just need to understand each other.
In your particular case I suggest to build a REST or RESTful API (https://flask-restful.readthedocs.org/en/0.3.3/) on the server and a REST client library on the client.
There are many ways and libraries to call REST APIs from C#:
The built-in method would be using HttpWebRequest as you can see on this link:
private async Task<JsonValue> FetchWeatherAsync (string url)
{
// Create an HTTP web request using the URL:
HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create (new Uri (url));
request.ContentType = "application/json";
request.Method = "GET";
// Send the request to the server and wait for the response:
using (WebResponse response = await request.GetResponseAsync ())
{
// Get a stream representation of the HTTP web response:
using (Stream stream = response.GetResponseStream ())
{
// Use this stream to build a JSON document object:
JsonValue jsonDoc = await Task.Run (() => JsonObject.Load (stream));
Console.Out.WriteLine("Response: {0}", jsonDoc.ToString ());
// Return the JSON document:
return jsonDoc;
}
}
}
But I don´t recommend it if you don´t want your app to be full of crap (boiler plate code) everywhere.
A helper library could be, for example, RESTSharp. It allows you to build REST calls easily and cast the response to your typed objects. Here´s and example:
var client = new RestClient("http://example.com");
// client.Authenticator = new HttpBasicAuthenticator(username, password);
var request = new RestRequest("resource/{id}", Method.POST);
request.AddParameter("name", "value"); // adds to POST or URL querystring based on Method
request.AddUrlSegment("id", "123"); // replaces matching token in request.Resource
// easily add HTTP Headers
request.AddHeader("header", "value");
// add files to upload (works with compatible verbs)
request.AddFile(path);
// execute the request
RestResponse response = client.Execute(request);
var content = response.Content; // raw content as string
// or automatically deserialize result
// return content type is sniffed but can be explicitly set via RestClient.AddHandler();
RestResponse<Person> response2 = client.Execute<Person>(request);
var name = response2.Data.Name;
// easy async support
client.ExecuteAsync(request, response => {
Console.WriteLine(response.Content);
});
// async with deserialization
var asyncHandle = client.ExecuteAsync<Person>(request, response => {
Console.WriteLine(response.Data.Name);
});
// abort the request on demand
asyncHandle.Abort();
You can search "C# REST client" on google and judge by yourself. But IMHO, the easier and nicer to code REST client I´ve ever used is Refit.
Why? you define API calls and responses with just an interface. No coding required at all! Even more, all your API calls will be async by default, something needed for mobile apps to be responsive. From the author´s readme:
public interface IGitHubApi
{
[Get("/users/{user}")]
Task<User> GetUser(string user);
}
var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com");
var octocat = await gitHubApi.GetUser("octocat");
I´ve used this library on Xamarin Android/iOS projects and it works well. No issues at all.
Hope it helps
I am developing a RESTFUL webservice using Django. On some occassion, we need to push the server object to the connected client without client polling.
We decided to use django-websocket 0.3.0.
I am writing the test cases and tried to connect to the server using nodejs ws client module
My View Function in Django is following:
from django.http import HttpResponse
from django_websocket import accept_websocket, require_websocket
from django.views.decorators.csrf import csrf_exempt
import json, sys, os, time, datetime
#csrf_exempt
#accept_websocket
def home(request) :
if not request.is_websocket():
return HttpResponse('new message')
else:
for message in request.websocket:
message = modify_message(message)
request.websocket.send(message)
request.websocket.close()
My Client Side code in js is like this:-
//Normal Get
var request = require('request');
request('http://127.0.0.1:8000',function(err,resp,flag){
console.log(resp.body);
});
//Opening a websocket
var WebSocket = require('ws');
var ws = new WebSocket('ws://127.0.0.1:8000/', {origin: 'http://127.0.0.1:8000'});
ws.on('open', function() {
console.log('connected');
ws.send(Date.now().toString(), {mask: true});
});
ws.on('close', function() {
console.log('disconnected');
});
ws.on('message', function(data, flags) {
console.log('Roundtrip time: ' + (Date.now() - parseInt(data)) + 'ms', flags);
setTimeout(function() {
ws.send(Date.now().toString(), {mask: true});
}, 500);
});
The first option gets the message as 'new message'
On the other side the second call throws the following error on the client side. On the server side, both commands pass through a 200OK
events.js:72
throw er; // Unhandled 'error' event
^
Error: unexpected server response (200)
at ClientRequest.<anonymous> (../ws/lib/WebSocket.js:603:17)
at ClientRequest.g (events.js:175:14)
at ClientRequest.EventEmitter.emit (events.js:95:17)
at HTTPParser.parserOnIncomingClient [as onIncoming] (http.js:1689:21)
at HTTPParser.parserOnHeadersComplete [as onHeadersComplete] (http.js:120:23)
at Socket.socketOnData [as ondata] (http.js:1584:20)
at TCP.onread (net.js:525:27)
On a side note if I log the request.is_websocket() on both calls it returns false meaning on the server side it never goes into the else part.
Please help me understand what mistake I am doing here
Thank you
Well,
I downloaded their entire code (not pip install) and run the supplied example chat program. Same error. The system sends a 400 response code for any ws:// call.
The git hub project page linked on the pypi site returns a 404 error. No way I can file a bug report. Emailed the developer and didn't get any response.
Probably something should have been broken on the new Django 1.5.2 version.
I consider that this is a dead project and hence moving to a complex but working solution like gevent or twisted.
thanks for your support!!!