如何在Django中创建在登录时设置会话变量的中间件？

from django.contrib.sessions.middleware import SessionMiddleware
from django.contrib.sessions.backends.db import SessionStore

class CustomSessionMiddleware(SessionMiddleware):
    def process_request(self, request):
        # 检查用户是否已登录
        if request.user.is_authenticated:
            # 设置会话变量
            request.session['custom_variable'] = 'custom_value'

MIDDLEWARE = [
    # 其他中间件...
    'myapp.session_middleware.CustomSessionMiddleware',
    # 其他中间件...
]