Before_request hooks in Flask - Step-by-Step Execution

Concept Flow - Before_request hooks

Client sends HTTP request

↓

Flask receives request

↓

Run before_request hooks

↓

Hook 1

↓

If hook returns response?

Yes→Send that response back

No↓

Run view function

↓

Send view response back

When Flask gets a request, it runs all before_request hooks first. If any hook returns a response, Flask sends it immediately. Otherwise, it runs the main view function.

Execution Sample

Flask

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

@app.before_request
def check_auth():
    if not request.headers.get('Auth'):
        return 'Unauthorized', 401

This code runs check_auth before every request. If 'Auth' header is missing, it stops and returns 401.

Execution Table

Step	Action	Request Headers	Hook check_auth result	Next Step	Response Sent
1	Receive request	{'Auth': 'token123'}	None	Run before_request hook	No
2	Run before_request hook	{'Auth': 'token123'}	None	Run view function	No
3	Run view function	{'Auth': 'token123'}	None	Send view response	No
4	Send response	{'Auth': 'token123'}	None	Done	Yes (view response)
5	Receive request	{} (no Auth)	Returns ('Unauthorized', 401)	Send hook response	Yes (401 Unauthorized)

💡 Execution stops early if before_request hook returns a response; otherwise, view function runs.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
request.headers	{}	{'Auth': 'token123'}	{'Auth': 'token123'}	{'Auth': 'token123'}	{'Auth': 'token123'}
hook_result	None	None	None	None	None
response_sent	No	No	No	Yes (view response)	Yes (view response)

Key Moments - 2 Insights

Why does Flask stop running the view function when a before_request hook returns a response?

Because the before_request hook returned a response (see execution_table step 5), Flask sends that response immediately and skips the view function.

What happens if no before_request hook returns a response?

Flask continues to run the view function normally (see execution_table steps 1-4), then sends the view's response.

Visual Quiz - 3 Questions

Look at the execution_table, what is the hook_result at step 2 when 'Auth' header is present?

A Error

B 'Unauthorized', 401

C None

D View function response

💡 Hint

Check the 'Hook check_auth result' column at step 2 in execution_table.

At which step does Flask send the response immediately due to a before_request hook?

A Step 3

B Step 5

C Step 1

D Step 4

💡 Hint

Look for the step where 'Response Sent' is 'Yes (401 Unauthorized)' in execution_table.

If the before_request hook never returns a response, what changes in the execution_table?

A View function runs and response is sent at step 4

B Flask skips the view function

C Hook result shows a response

D Response is sent at step 5

💡 Hint

Refer to the normal flow rows where hook_result is None and view function runs.

Concept Snapshot

Flask before_request hooks run before each request.
If a hook returns a response, Flask sends it immediately.
Otherwise, Flask runs the view function.
Use @app.before_request decorator.
Common for auth checks or setup.
Stops request early if needed.

Full Transcript

In Flask, before_request hooks run before the main view function for every incoming request. If any hook returns a response, Flask immediately sends that response and skips the view function. Otherwise, the view function runs normally and its response is sent. This lets you run checks or setup code before handling requests. For example, you can check if an authorization header exists and return a 401 error early if missing. The execution table shows how Flask processes requests step-by-step, running hooks first, then the view, and sending the response. If a hook returns a response, Flask stops early and sends it. This helps control request flow simply and clearly.