WSGI middleware concept in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a WSGI middleware class that wraps an app.

Flask

class SimpleMiddleware:
    def __init__(self, app):
        self.app = [1]

    def __call__(self, environ, start_response):
        return self.app(environ, start_response)

Drag options to blanks, or click blank then click option'

Aenviron

Bapp

Cstart_response

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the wrong variable to self.app

Using environ or start_response instead of app

The middleware stores the wrapped WSGI app in self.app. So the constructor must assign app to self.app.

2fill in blank

medium

Complete the code to call the wrapped app inside the middleware __call__ method.

Flask

def __call__(self, environ, start_response):
    # Call the wrapped app
    return self.app([1], start_response)

Drag options to blanks, or click blank then click option'

Aenviron

Bself

Cstart_response

Dapp

Attempts:

3 left

💡 Hint

Common Mistakes

Passing self or start_response as the first argument

Swapping the order of arguments

The WSGI app is called with the environ dictionary and the start_response callable.

3fill in blank

hard

Fix the error in the middleware call by completing the code to modify the response headers.

Flask

def __call__(self, environ, start_response):
    def custom_start(status, headers, exc_info=None):
        headers.append(('X-Custom', 'Middleware'))
        return start_response(status, headers, exc_info)

    return self.app(environ, [1])

Drag options to blanks, or click blank then click option'

Astart_response

Bself

Cenviron

Dcustom_start

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the original start_response instead of custom_start

Passing environ instead of a callable

The middleware replaces the start_response callable with custom_start to add headers before calling the original start_response.

4fill in blank

hard

Fill both blanks to create a middleware that logs the request path and then calls the app.

Flask

def __call__(self, environ, start_response):
    print('Request path:', environ[[1]])
    return self.app(environ, [2])

Drag options to blanks, or click blank then click option'

A'PATH_INFO'

Bstart_response

C'REQUEST_METHOD'

Dcustom_start

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'REQUEST_METHOD' instead of 'PATH_INFO' for the path

Passing custom_start when it is not defined

The request path is in environ['PATH_INFO']. The middleware then calls the wrapped app with start_response.

5fill in blank

hard

Fill all three blanks to create a middleware that adds a header and logs the method.

Flask

def __call__(self, environ, start_response):
    def [1](status, headers, exc_info=None):
        headers.append(('X-Added-By', 'Middleware'))
        return start_response(status, headers, exc_info)

    print('Method:', environ[[2]])
    return self.app(environ, [3])

Drag options to blanks, or click blank then click option'

Acustom_start

B'REQUEST_METHOD'

Dstart_response

Attempts:

3 left

💡 Hint

Common Mistakes

Using start_response instead of custom_start when calling the app

Using 'PATH_INFO' instead of 'REQUEST_METHOD' for the method

The middleware defines a custom_start function to add headers. It logs the HTTP method from environ['REQUEST_METHOD'] and calls the app with custom_start.