Global variable is None instead of instance - Python

问题

I'm dealing with global variables in Python. The code should work fine, but there is a problem. I have to use global variable for instance of class Back. When I run the application it says that back is None which should be not true because the second line in setup() function - 'back = Back.Back()'

# -*- coding: utf-8 -*-
from flask import Flask
from flask import request
from flask import render_template


import Search
import Back

app = Flask(__name__)
global back
back = None

@app.route('/')
def my_form():
    return render_template('my-form.html')

def setup():
    global back
    back = Back.Back()

def is_ascii(s):
    return all(ord(c) < 128 for c in s)  

@app.route('/', methods=['POST'])
def search():
    from time import time

    pattern = request.form['text']

    startTime = time()

    pattern=pattern.lower()

    arr = []

    if len(pattern)<1:
        arr.append('Incorrect input')

        currentTime = time()-startTime

        return render_template('my-form.html', arr=arr, time=currentTime)


    arr = []

    search = Search.Search(pattern,back)
    results =  search.getResults()

    ..................

    return render_template('my-form.html', arr=arr, time=currentTime, pattern=pattern)


app.debug=True
if __name__ == '__main__':
    setup()
    app.run()

Why is the back variable None instead of instance of Back class? Thanks

回答1:

The Flask development server runs your module twice. Once to run the server itself, and another time in a child process so it can reload your whole script each time you make a change to it. It is that second process that won't run the __main__ guarded code and the global is left as None.

You'll get the same problem if you used another WSGI server; it'd import your file as a module, not as the initial script and the __main__ guard is not executed.

Use a @app.before_first_request function instead; it is guaranteed to be executed when the very first request is handled for this process. This also keeps your global working if you moved to a proper WSGI container that used multiple child processes to scale your site:

@app.before_first_request
def setup():
    global back
    back = Back.Back()

来源：https://stackoverflow.com/questions/28238478/global-variable-is-none-instead-of-instance-python

标签

python

global-variables

nonetype