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?
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！