在使用Flask-SocketIO制作WebSocket应用程序，结果程序启动失败

Question

问题描述：

• 在使用Flask-SocketIO制作WebSocket的过程中，发现不安装eventlet模块是无法启用WebSocket的，只能使用Ajax轮询（Ajax Polling），占用服务器带宽。
• 但是，经测试，安装eventlet之后程序不仅无法使用WebSocket，同时还启动不了。通过Flask-SocketIO官方Github提交Issue的Problems分类里的提示，在SocketIO的对象创建时传入两个参数（logger=True, engineio_logger=True），结果日志就这么一行：

Server initialized for eventlet.

• 证明，错误不在eventlet上......求大佬解释一下这个情况......

代码主程序贴在这里：

# coding: utf-8

from flask import Flask, redirect, render_template, request
from flask_socketio import SocketIO  # 连接
import requests

app = Flask("app")


io = SocketIO(app, logger=True, engineio_logger=True)


@app.route("/")
def index_redirect_handler():
    return redirect("/ui")


@app.route("/ui")
def index_ui():
    return render_template("index.html")


@app.route("/dl", methods=['POST'])
def index_download_file():
    return render_template(
        "download.html",
        **request.form.to_dict()
    ).replace("__url__", request.form.get("url")) \
     .replace("__path__", request.form.get("path"))


@io.on("download")
def download_file(data):
    def upload_status(status):
        io.emit("status", {
            "status": status
        })
        return

    response: requests.Response = requests.get(
        url=data['url'],
        stream=True,
        headers={
            "User-Agent": "Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/"
            "537.36 (KHTML, like Gecko) Chrome/79.0.3282.204 Safari/537.36"
        }
    )
    if response.status_code != 200:
        io.emit(
            "status_code",
            {
                "code": str(response.status_code)
            }
        )
        io.stop()
        return
    size = 0
    print(response.headers)
    try:
        content_size = float(response.headers['Content-Length'])
        io.emit(
            "file_size",
            {
                "size": str(content_size / 1024 / 1024) + "MB"
            }
        )
    except KeyError:
        content_size = None
        io.emit(
            "file_size",
            {
                "size": "unknown"
            }
        )
    chunk_size = 1024
    f = open(data['path'], "wb")
    for data in response.iter_content(chunk_size=chunk_size):
        size += len(data)
        if content_size is not None:
            upload_status("%f%%" % (size / content_size * 100))
        f.write(data)
    f.close()
    io.emit("success")
    return


# app.run("0.0.0.0", 8006)
# 使用io.run可以同时启用socketio服务端
io.run(app, "0.0.0.0", 8006)