Falcon proxying example using a sink (tested with Falcon 2.0.0rc4 and CPython 3.7)

proxy.py

import io

import falcon
import requests


class Proxy(object):
    """Try, for instance ``/examples/forms1.html``."""

    UPSTREAM = 'https://www.simplehtmlguide.com'

    def __init__(self):
        self.session = requests.Session()

    def handle(self, req, resp):
        headers = dict(req.headers, Via='Falcon')
        for name in ('HOST', 'CONNECTION', 'REFERER'):
            headers.pop(name, None)

        request = requests.Request(req.method, self.UPSTREAM + req.path,
                                   data=req.bounded_stream.read(),
                                   headers=headers)
        prepared = request.prepare()
        from_upstream = self.session.send(prepared, stream=True)

        resp.content_type = from_upstream.headers.get('Content-Type',
                                                      falcon.MEDIA_HTML)
        resp.status = falcon.get_http_status(from_upstream.status_code)
        resp.stream = from_upstream.iter_content(io.DEFAULT_BUFFER_SIZE)


api = falcon.API()
api.add_sink(Proxy().handle)

Exercise for the reader: stream request directly from req.bounded_stream, do not buffer it in memory with .read().