为什么运行 Flask 开发服务器会自己运行两次? [英] Why does running the Flask dev server run itself twice?

问题描述

我正在使用 Flask 开发网站，在开发过程中我使用以下文件运行烧瓶:

I'm using Flask for developing a website and while in development I run flask using the following file:

#!/usr/bin/env python
from datetime import datetime
from app import app
import config

if __name__ == '__main__':
    print('################### Restarting @', datetime.utcnow(), '###################')
    app.run(port=4004, debug=config.DEBUG, host='0.0.0.0')

当我启动服务器时，或者当它因为文件更新而自动重启时，它总是显示两次打印行:

When I start the server, or when it auto-restarts because files have been updated, it always shows the print line twice:

################### Restarting @ 2014-08-26 10:51:49.167062 ###################
################### Restarting @ 2014-08-26 10:51:49.607096 ###################

虽然这不是一个真正的问题(其余的按预期工作)，但我只是想知道它为什么会这样?有什么想法吗?

Although it is not really a problem (the rest works as expected), I simply wonder why it behaves like this? Any ideas?

推荐答案

Werkzeug 重新加载器产生一个子进程，以便它可以在每次代码更改时重新启动该进程.Werkzeug 是在您调用 app.run() 时为 Flask 提供开发服务器的库.

The Werkzeug reloader spawns a child process so that it can restart that process each time your code changes. Werkzeug is the library that supplies Flask with the development server when you call app.run().

参见_reloader 功能代码;您的脚本使用 subprocess.call()再次 运行.

See the restart_with_reloader() function code; your script is run again with subprocess.call().

如果您将 use_reloader 设置为 False，您将看到行为消失，但同时您也失去了重新加载功能:

If you set use_reloader to False you'll see the behaviour go away, but then you also lose the reloading functionality:

app.run(port=4004, debug=config.DEBUG, host='0.0.0.0', use_reloader=False)

您也可以在使用 flask run 命令时禁用重载器:

You can disable the reloader when using the flask run command too:

FLASK_DEBUG=1 flask run --no-reload

您可以使用 werkzeug.serving.is_running_from_reloader 函数 如果你想检测你何时处于重载子进程:

You can use the werkzeug.serving.is_running_from_reloader function if you wanted to detect when you are in the reloading child process:

from werkzeug.serving import is_running_from_reloader

if is_running_from_reloader():
    print(f"################### Restarting @ {datetime.utcnow()} ###################")

但是，如果您需要设置模块全局变量，那么您应该使用 @app.before_first_request 装饰器 并让该函数设置这样的全局变量.当第一个请求进来时，它会在每次重新加载后只调用一次:

However, if you need to set up module globals, then you should instead use the @app.before_first_request decorator on a function and have that function set up such globals. It'll be called just once after every reload when the first request comes in:

@app.before_first_request
def before_first_request():
    print(f"########### Restarted, first request @ {datetime.utcnow()} ############")

请注意，如果您在使用分叉或新子进程处理请求的全尺寸 WSGI 服务器中运行它，before_first_request 处理程序可能被调用每个新的子流程.

Do take into account that if you run this in a full-scale WSGI server that uses forking or new subprocesses to handle requests, that before_first_request handlers may be invoked for each new subprocess.

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