Example WSGI app and middleware for What the Heck is a WSGI Presentation

main.py

from io import BytesIO
from wsgiref.simple_server import make_server


def app(environ, start_response):
    try:
        content_length = int(environ["CONTENT_LENGTH"])
        name = environ["wsgi.input"].read(content_length)
    except (KeyError, ValueError):
        name = b"World"
    status = "200 OK"
    response_headers = [("Content-Type", "text/plain")]
    start_response(status, response_headers)
    return [b"Hello, ", name, b"!"]


class StartResponseCapture:
    def __init__(self):
        self.status = None
        self.headers = {}

    def __call__(self, status, headers):
        self.status = status
        self.headers = headers


class CachingMiddleware:
    def __init__(self, next_):
        self.__next = next_
        self.__cache = {}

    def __call__(self, environ, start_response):
        try:
            content_length = int(environ["CONTENT_LENGTH"])
            wsgi_input = environ["wsgi.input"].read(content_length)
            environ["wsgi.input"] = BytesIO(wsgi_input)
        except (KeyError, ValueError):
            wsgi_input = b""

        if wsgi_input in self.__cache:
            cached = True
            cached_data = self.__cache[wsgi_input]
            status = cached_data["status"]
            headers = cached_data["headers"]
            response = cached_data["response"]
        else:
            cached = False
            start_response_capture = StartResponseCapture()
            result = self.__next(environ, start_response_capture)
            status = start_response_capture.status
            headers = start_response_capture.headers
            response = [chunk for chunk in result]
            self.__cache[wsgi_input] = {
                "status": status,
                "headers": headers,
                "response": response,
            }
        start_response(status, headers + [("X-Cache-Hit", str(cached))])
        return response


caching_app = CachingMiddleware(app)

if __name__ == '__main__':
    with make_server('127.0.0.1', 5000, caching_app) as httpd:
        print("Serving HTTP on port 5000...")
        httpd.serve_forever()