Before_request as middleware alternative in Flask - Step-by-Step Execution

Concept Flow - Before_request as middleware alternative

Client sends HTTP request

↓

Flask app receives request

↓

@before_request function runs

↓

Continue to route

↓

Route handler runs

↓

Response sent back to client

This flow shows how Flask runs a function before each request to act like middleware, deciding to continue or return early.

Execution Sample

Flask

from flask import Flask, request
app = Flask(__name__)

@app.before_request
def check_auth():
    if request.headers.get('X-Auth') is None:
        return 'Unauthorized', 401

This code runs check_auth before every request to block unauthorized access if header missing.

Execution Table

Step	Action	Request Headers	before_request Result	Route Handler Called	Response Sent
1	Request received with headers {'X-Auth': 'token123'}	{'X-Auth': 'token123'}	None (continue)	Yes	Route response
2	Request received with headers {}	{}	'Unauthorized', 401 (stop)	No	'Unauthorized', 401
3	Request received with headers {'X-Auth': ''}	{'X-Auth': ''}	None (continue)	Yes	Route response

💡 Execution stops early if before_request returns a response, otherwise route handler runs.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
request.headers	{}	{'X-Auth': 'token123'}	{}	{'X-Auth': ''}
before_request return	None	None	'Unauthorized', 401	None
route_handler_called	False	True	False	True
response_sent	None	Route response	'Unauthorized', 401	Route response

Key Moments - 2 Insights

Why does the route handler not run when before_request returns a response?

Because returning a response from before_request stops further processing, as shown in execution_table row 2 where route_handler_called is No.

What happens if before_request returns None?

Returning None means continue processing normally, so the route handler runs, as seen in rows 1 and 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the before_request return value at step 2?

A 'OK', 200

B None

C 'Unauthorized', 401

D Raises error

💡 Hint

Check the 'before_request Result' column at step 2 in the execution_table.

At which step does the route handler NOT get called?

A Step 2

B Step 1

C Step 3

D All steps call route handler

💡 Hint

Look at the 'Route Handler Called' column in the execution_table.

If the request headers always include 'X-Auth', how would the response_sent column change?

A It would always be 'Unauthorized', 401

B It would always be 'Route response'

C It would alternate between 'Unauthorized' and 'Route response'

D It would be empty

💡 Hint

Refer to variable_tracker for response_sent values when 'X-Auth' header is present.

Concept Snapshot

Flask @before_request runs a function before each request.
If it returns a response, request stops and response is sent early.
If it returns None, request continues to route handler.
Use it like middleware to check auth or modify requests.
Helps centralize pre-route logic in one place.

Full Transcript

In Flask, the @before_request decorator lets you run a function before every request. This function can check things like authentication headers. If it returns a response, Flask sends that response immediately and skips the route handler. If it returns None, Flask continues to the route handler as usual. This acts like middleware, letting you centralize checks or setup before routes run. For example, checking for an 'X-Auth' header and returning 'Unauthorized' if missing stops the request early. Otherwise, the route runs normally. This flow helps keep your app organized and secure.