AppContext vs. RequestContext and Flask vs. Request

In Flask, there are two types of context:

1. AppContext, it’s a Flask object

app = Flask(__name__)

2. RequestContext, it’s a Request object

• Flask core object is stored in AppContext

{% asset_img "AppContext.PNG" %}

• Request object is stored in RequestContext

{% asset_img "RequestContext.PNG" %}

Why is context needed?

It’s a type of system design. For certain objects, some info belongs to the external environment not the objects themselves.
But if we want to use this info with e.g. the core Flask object, we can put both Flask and external info into a new object,
called AppContext.

To clarify:

• Flask: core object. It contains the core features in Flask e.g. routes, config

• AppContext: an encapsulation of Flask with additional external parameters

• Request: core object. It contains the request info, url parameters etc.

• RequestContext: an encapsulation of Request, similar to AppContext

We need to access Flask and Request via AppContext and RequestContext, however in the code we rarely use AppContext or RequestContext explicitly.

Because LocalProxy has the ability to operate on context objects indirectly.

{% asset_img "localproxy.PNG" %}

How is context operated?

1. Start with a request
2. Flask will first check the top of _app_ctx_stack, if it’s None or not the same as the current_app,
   an AppContext will be pushed into _app_ctx_stack; then push RequestContext to _request_ctx_stack
3. Once the request is ended, RequestContext is popped

for example:

from flask import Flask, current_app

app = Flask(__name__)

ctx = app.app_context()
ctx.push()
a = current_app
d = current_app.config["DEBUG"]
ctx.pop()

• We can apply with to the context objects, aka objects with __enter__ and __exit__ methods

• The above can be re-written as below

from flask import Flask, current_app

app = Flask(__name__)

with app.app_context():
    b = current_app
    c = current_app.config["DEBUG"]

• AppContext or RequestContext are essentially context managers

• with app.app_context() is the context expression, which need to return a context manager

• with is an efficient way to manage resources