如何避免使用全局变量？ （python-flask-socketio应用）

Question

I'm trying to figure out how to not use global variables for my application but I can't think of anything else.

I'm actually coding a web interface with the help of the Flask-SocketIO module to interact in real time with a music player.

This is a snippet of my code containing the play function (I think I only need one example and then I can adapt it for all the other functions):

from flask import Flask, render_template
from flask_socketio import SocketIO

app = Flask(__name__)
socketio = SocketIO(app)
isPlaying = False #This is the variable that I would like to avoid making global

@socketio.on('request_play')
def cycle_play():
    global isPlaying
    if isPlaying == True:
        socketio.emit('pause', broadcast=True)
        isPlaying = False
    else:
        socketio.emit('play', broadcast=True)
        isPlaying = True

if __name__ == '__main__':
    socketio.run(app, port=5001)

This is only a stripped down version of the code but I think it's enough to understand what I'm trying to accomplish.

I need to access that variable also from other functions and I need to do the same with the song name, duration and current time.

Thanks in advance for the help and sorry if my English is not clear.

---

Here is the solution that I used:

from flask import Flask, render_template
from flask_socketio import SocketIO

app = Flask(__name__)
socketio = SocketIO(app)

class Player():
    def __init__(self):
        self.isPlaying = False


    def cycle_play(self):
        if self.isPlaying == True:
            socketio.emit('pause', broadcast=True)
            self.isPlaying = False
        else:
            socketio.emit('play', broadcast=True)
            self.isPlaying = True


if __name__ == '__main__':
    player = Player()
    socketio.on('request_play')(player.cycle_play) #this is the decorator
    socketio.run(app, port=5001)

Answer 1

You can use the user session to store such values. You can read up more about the session object here: flask.pocoo.org/docs/0.12/quickstart/#sessions.

from flask import session

@socketio.on('initialize')
def initialize(isPlaying):
    session['isPlaying'] = isPlaying

@socketio.on('request_play')
def cycle_play():
    # Note, it's good practice to use 'is' instead of '==' when comparing against builtin constants.
    # PEP8 recommended way is to check for trueness rather than the value True, so you'd want to first assert that this variable can only be Boolean.
    assert type(session['isPlaying']) is bool

    if session['isPlaying']: 
        socketio.emit('pause', broadcast=True)
        session['isPlaying'] = False
    else:
        socketio.emit('play', broadcast=True)
        session['isPlaying'] = True

Answer 2

The solution that suggests itself is to define a class which encapsulates both the state variable and the responses to its change. Since I'm not familiar with the details of Flask-SocketIO please treat this as pseduocode rather than something to be pasted in to a working program.

class PlayControl:
    def __init__(self, initial):
        self.is_playing = initial
    def cycle_play(self):
        if self.is_playing:
            socketio.emit('pause', broadcast=True)
            self.is_playing = False
    else:
            socketio.emit('play', broadcast=True)
            self.is_playing = True

You would then create an instance of this class, and pass the instance's cycle_play method to the same function you decorated your original function with. Because this behaviour is dynamic it's not appropriate to use a decorator on the method definition.

control = PlayControl(False)
socketio.on('request_play')(control.cycle_play)

To reduce the amount of program code you could even define a class that took the functions to call and the values to emit as arguments, generalising the concept further to make code more concise and with less boilerplate.

Answer 3

My advice would be use a class and inside init method just use self.isPlaying = False. You can always refer to this variable from all the function in the class. for exemple:

class PLAYER(object):   
   def __init__(self,other parameters):
    self.isPlaying = False
    #your cod
   def cycle_play(self):
        #global isPlaying
        if self.isPlaying == True:
           socketio.emit('pause', broadcast=True)
           self.isPlaying = False
        else:
           socketio.emit('play', broadcast=True)
           self.isPlaying = True