[Solved] Why does this middleware cause an error?class ErrorMiddleware: def __init__(self, app): self.app = app def __call__(self, environ, start_response): start_response('200 OK') return self.... | Flask

Flask - Middleware and Extensions

Why does this middleware cause an error?

class ErrorMiddleware:
    def __init__(self, app):
        self.app = app
    def __call__(self, environ, start_response):
        start_response('200 OK')
        return self.app(environ, start_response)

app.wsgi_app = ErrorMiddleware(app.wsgi_app)

AMissing return statement in __call__

Bapp.wsgi_app not wrapped properly

CMiddleware class missing __init__ method

Dstart_response called twice causing protocol error

Step-by-Step Solution

Solution:

Step 1: Understand start_response usage
start_response must be called once per request to set headers.
Step 2: Identify double call
Middleware calls start_response, then wrapped app calls it again causing error.
Final Answer:
start_response called twice causing protocol error -> Option D
Quick Check:
Call start_response only once per request [OK]

Quick Trick: Call start_response only once in middleware chain [OK]

Common Mistakes:

MISTAKES

Calling start_response before wrapped app
Missing return statement
Not wrapping app.wsgi_app

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Why does this middleware cause an error?

Step 1: Understand start_response usage

Step 2: Identify double call

Final Answer:

Quick Check:

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