python flask全局变量问题

问题描述：当在python的flask中定义全局变量之后，当其他模块修改全局变量，flask中的api运行时读取的全局变量值还是定义时的初始值。
在https://stackoverflow.com/questions/23457658/flask-global-variables有人遇到了同样的问题。

I am trying to find out how to work with global variables in Flask:  gl = {'name': 'Default'}  @app.route('/store/<name>') def store_var(name=None):  gl['name'] = name  return "Storing " + gl['name']  @app.route("/retrieve") def retrieve_var():  n = gl['name']  return "Retrieved: " + n Storing the name and retrieving it from another client works fine. However, this doesn't feel right: a simple global dictionary where any session pretty much simultaneously can throw in complex objects, does that really work without any dire consequences? 
                    

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下面有人说了导致问题的原因：

No, it doesn't work, not outside the simple Flask development server.  WSGI servers scale in two ways; by using threads or by forking the process. A global dictionary is not a thread-safe storage, and when using multi-processing changes to globals are not going to be shared. If you run this on a PAAS provider like Google App Server, the processes aren't even forked; they are run on entirely separate machines even. 

可能的解决方法：
Use some kind of backend storage instead; a memcached server, a database server, something to control concurrent access and share the data across processes.
另一个可能的解决方法：
参考：https://www.cnpython.com/qa/78502

你不需要一个全局变量来实现这一点，并且说真的，你在未来的任何时候都不需要它们，因为使用全局变量是一种不好的做法。有关详细信息，请参阅此链接，why are global variables evil?
现在谈到您的问题，您可能需要flask的g模块，该模块创建一个上下文，该上下文在同一用户的多个请求上保持不变。你可以这样做：

from flask import g  ...  def get_messages():  messages = getattr(g, '_messages', None)  if messages is None:  g._messages = [] # to store messages you may use a dictionary   return g._messages  def add_message(message):  messages = get_messages()  messages.append(message)  setattr(g, '_messages', messages)  return messages 

记住，对于每个用户，应用程序都会创建一个不同的线程，这样既不会共享变量，也不会共享它们的值。因此，对于每个用户，都有一个不同的g，但它将在同一个用户的多个请求上持续存在。希望有帮助！