flask/docs/patterns/methodoverrides.rst

Adding HTTP Method Overrides
============================

Some HTTP proxies do not support arbitrary HTTP methods or newer HTTP
methods (such as PATCH).  In that case it's possible to “proxy” HTTP
methods through another HTTP method in total violation of the protocol.

The way this works is by letting the client do an HTTP POST request and
set the ``X-HTTP-Method-Override`` header and set the value to the
intended HTTP method (such as ``PATCH``).

This can easily be accomplished with an HTTP middleware::

    class HTTPMethodOverrideMiddleware(object):
        allowed_methods = frozenset([
            'GET',
            'HEAD',
            'POST',
            'DELETE',
            'PUT',
            'PATCH',
            'OPTIONS'
        ])
        bodyless_methods = frozenset(['GET', 'HEAD', 'OPTIONS', 'DELETE'])

        def __init__(self, app):
            self.app = app

        def __call__(self, environ, start_response):
            method = environ.get('HTTP_X_HTTP_METHOD_OVERRIDE', '').upper()
            if method in self.allowed_methods:
                method = method.encode('ascii', 'replace')# Remove this line if you are using Python3. 
                environ['REQUEST_METHOD'] = method
            if method in self.bodyless_methods:
                environ['CONTENT_LENGTH'] = '0'
            return self.app(environ, start_response)

To use this with Flask this is all that is necessary::

    from flask import Flask

    app = Flask(__name__)
    app.wsgi_app = HTTPMethodOverrideMiddleware(app.wsgi_app)