How does Context Locals and The Request Context work together?

Question

I am trying to learn to make a variable available across method through a decorator function in Flask.

I read Flask request context documentation and wrote the following code, which works as intended.

a.py

_request_ctx_stack.top.current_identity = payload.get('sub')

b.py

current_identity = getattr(_request_ctx_stack.top, 'current_identity', None)

However flask-jwt solves this problem by introducing an additional local proxy like this:

a.py

from werkzeug.local import LocalProxy

current_identity = LocalProxy(lambda: getattr(_request_ctx_stack.top, 'current_identity', None))
_request_ctx_stack.top.current_identity = payload.get('sub')

b.py

from a import current_identity

Why? I read werkzeug context locals documentation and doesn't Flask already implements Werkzeug context locals for request object?

Is there any advantage of introducing LocalProxy?

Answer 1

The LocalProxy wraps the manual code you wrote for getting the value. Instead of needing that manual code everywhere you want to access current_identity, you access it as a proxy and it does the manual code for you.

It's most useful in libraries, where users wouldn't be expected to know how current_identity is set up, and would import the proxy to access it. The same applies to Flask itself: you're not expected to know how the request is set up, or where exactly it's stored, only that you access it by importing request.

A proxy is useful for data that is set up during and local to each request. If you used a true global variable, it would not behave the way you expect when multiple requests are being handled. See Are global variables thread safe in flask? How do I share data between requests?