无限运行 Flask 应用程序

Question

I am running a simple website which shows the current price of silver. How do I make it run in an infinite loop, so that it keeps updating the price after scraping it from silverseek.com after any interval(10secs for eg? I tried working with BackgroundScheduler, however, it throws an error

Execution of job "silver (trigger: interval[0:00:10], next run at: 2020-01-08 17:56:27 IST)" skipped: maximum number of running instances reached (1) Job "silver (trigger: interval[0:00:10], next run at: 2020-01-08 17:56:37 IST)" raised an exception Traceback (most recent call last): File "C:\\Users\\Abhi\\PycharmProjects\\Cricket\\venv\\lib\\site-packages\\apscheduler\\executors\\base.py", line 125, in run_job retval = job.func(*job.args, **job.kwargs) File "C:/Users/Abhi/PycharmProjects/Cricket/internet.py", line 25, in silver return render_template('home.html', s=txt.get_text()) File "C:\\Users\\Abhi\\PycharmProjects\\Cricket\\venv\\lib\\site-packages\\flask\\templating.py", line 136, in render_template ctx.app.update_template_context(context) AttributeError: 'NoneType' object has no attribute 'app'

Here is the code.

from bs4 import BeautifulSoup
from selenium import webdriver
import time
from apscheduler.schedulers.background import BackgroundScheduler
from flask import Flask, render_template




app = Flask(__name__)


@app.route('/')
def silver():
    url = "http://silverseek.com/"
    chrome_options = webdriver.ChromeOptions()
    chrome_options.add_argument('--headless')
    driver = webdriver.Chrome(executable_path='C:/Users/Rags/Downloads/cd79/chromedriver.exe', options=chrome_options)
    driver.get(url)
    time.sleep(8)
    sil = driver.page_source
    bs = BeautifulSoup(sil, "lxml")
    elem = bs.find_all('div', class_='quote-container')
    for txt in elem:
        return render_template('home.html', s=txt.get_text())


sched = BackgroundScheduler(daemon=True)
sched.add_job(silver, 'interval', seconds=10)
sched.start()

if __name__ == "__main__":
    app.run()

Answer 1

You mean that you hope the page content to automagically refresh in your browser ? – bruno desthuilliers 19 mins ago
@brunodesthuilliers Yes, that was the idea..I hope I am not entirely wrong in some way.

Mmm... That's what I suspected - and that's not how it works at all. The HTTP protocol is based on a request / response cycle - the client send a request (to GET a given url, or POST some data at a given URL etc), the request is routed to the server, the server returns a response (with the status code, content and headers), AND THE CYCLE IS OVER. The server doesn't keep any connection to the client once the response has been sent.

Actually, your app IS already "running forever" - that's what the app.run() calls do - waiting for requests to come in. Each time you send a request (each time you load the page in your browser for example), your silver() function is called and the result is returned as a HTTP response (flask takes care of wrapping it in a proper response if necessary). Of course once the browser has rendered the response (displaying your "page"), nothing else happens - the page will not magically refresh itself. But if you manually refresh it (reload the page - F5 key for most browsers), it will send a new request, and you will then get up to date informations.

If you want your page to keep on refreshing itself, you either have to make the client (in this case your browser) refrehsing it, or use a more elaborate protocol like websockets etc (something that maintains a connection between client and server so the server can push to the client).

For your case, this is certainly overkill, so the simple obvious solution is to use client-side scripting - IOW javascript - to refresh the page every X seconds. This is trivial and there's no shortage of examples so I won't bother posting one here.

Now if this is for learning purpose, you may still want to try out the "more elaborate" solution too so you get a first taste of how it works (django has 'django-channels' for this, and there's very certainly something similar for flask), but I reckon you'd be better learning more about HTTP itself first before moving on to those more advanced topics.