Custom middleware creation in Flask - Step-by-Step Execution

Concept Flow - Custom middleware creation

Request comes in

↓

Middleware processes request

↓

Pass request to Flask app

↓

Flask app processes request

↓

Response generated

↓

Middleware processes response

↓

Response sent back to client

Middleware intercepts requests before Flask handles them and can modify requests or responses.

Execution Sample

Flask

from flask import Flask, request

class SimpleMiddleware:
    def __init__(self, app):
        self.app = app
    def __call__(self, environ, start_response):
        print('Middleware before app')
        return self.app(environ, start_response)

This code defines a simple middleware class that prints a message before passing the request to the Flask app.

Execution Table

Step	Action	Middleware Output	Flask App Output	Result
1	Request received by middleware	Prints 'Middleware before app'		Request passed to app
2	Flask app processes request		Generates response 'Hello World!'	Response created
3	Middleware receives response	Can modify response (not shown here)		Response sent to client
4	Request cycle ends			Response delivered to client

💡 Request processed and response sent back to client

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
environ	Request environment dict	Passed unchanged	Passed unchanged	Passed unchanged	Used to build response
start_response	Callable to start response	Passed unchanged	Passed unchanged	Passed unchanged	Called to send response
response	None	None	Response object created	Possibly modified	Sent to client

Key Moments - 2 Insights

Why does middleware need to call the Flask app inside __call__?

Because middleware wraps the app, it must call the app with environ and start_response to let Flask handle the request, as shown in execution_table step 1.

Can middleware modify the response before sending it back?

Yes, middleware can modify the response after the app returns it, as indicated in execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution table, what does the middleware print when a request is received?

A Middleware before app

B Flask app started

C Response generated

D Request ended

💡 Hint

Check execution_table row 1 under Middleware Output

At which step does the Flask app generate the response?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at execution_table row 2 under Flask App Output

If middleware did not call the Flask app, what would happen?

A Request would be processed normally

B Request would not reach the Flask app and no response would be generated

C Middleware would generate the response automatically

D Response would be sent twice

💡 Hint

Refer to key_moments about why middleware must call the app

Concept Snapshot

Custom middleware wraps the Flask app.
It intercepts requests and responses.
Middleware __call__ method receives environ and start_response.
Middleware must call the app to continue processing.
Can modify request or response.
Useful for logging, auth, or headers.

Full Transcript

Custom middleware in Flask is a way to intercept requests before they reach the app and responses before they go back to the client. The middleware is a class with a __call__ method that receives the request environment and a start_response callable. It can do things like print messages or modify data. The middleware must call the Flask app inside __call__ to let the app process the request and generate a response. After the app returns, middleware can also modify the response before sending it back. This flow ensures middleware can add features like logging or authentication without changing the app code.