Certain proxy servers require the Full HTTP request to be included in the same packet as the HTTP CONNECT, however requests seems to split these up into multiple packets. This work around combines the headers and connect into a single send(). Documented here: https://github.com/requests/requests/issues/4884

requests_connect_with_headers.py

"""
requests_connect_with_headers.py
@brad_anton

Certain proxy servers require the Full HTTP request to be included in 
the same packet as the HTTP CONNECT, however requests seems to split 
these up into multiple packets. This work around combines the headers 
and connect into a single send(). 

Documented here: https://github.com/requests/requests/issues/4884 
"""

from requests import Session
from requests.adapters import HTTPAdapter
from requests.packages.urllib3 import proxy_from_url
from requests.packages.urllib3.connection import VerifiedHTTPSConnection

import logging

try:
    import http.client as http_client
    from http.HTTPStatus import OK
except ImportError:
    # Python 2
    import httplib as http_client
    from httplib import OK

http_client.HTTPConnection.debuglevel = 1

class VerifiedHTTPSConnectionWithHeaders(VerifiedHTTPSConnection):
    def _tunnel(self):
        """This is just a simple rework of the CONNECT method to combine
        the headers with the CONNECT request as it causes problems for
        some proxies
        """
        connect_str = "CONNECT %s:%d HTTP/1.0\r\n" % (self._tunnel_host,
            self._tunnel_port)
        header_bytes = connect_str.encode("ascii")

        for header, value in self._tunnel_headers.items():
            header_str = "%s: %s\r\n" % (header, value)
            header_bytes += header_str.encode("latin-1")

        self.send(header_bytes + b'\r\n')

        response = self.response_class(self.sock, method=self._method)
        (version, code, message) = response._read_status()

        if code != OK:
            self.close()
            raise OSError("Tunnel connection failed: %d %s" % (code,
                                                               message.strip()))
        while True:
            line = response.fp.readline(http_client._MAXLINE + 1)
            if len(line) > http_client._MAXLINE:
                raise LineTooLong("header line")
            if not line:
                # for sites which EOF without sending a trailer
                break
            if line in (b'\r\n', b'\n', b''):
                break

            if self.debuglevel > 0:
                print('header:', line.decode())

class ProxyConnectWithHeadersHTTPSAdapter(HTTPAdapter):
    """Overriding HTTP Adapter so that we can use our own Connection, since
        we need to get at _tunnel()
    """
    def proxy_manager_for(self, proxy, **proxy_kwargs):
        manager = super(ProxyConnectWithHeadersHTTPSAdapter, self).proxy_manager_for(proxy, **proxy_kwargs)

        # Need to override the ConnectionCls with our Subclassed one to get at _tunnel()
        manager.pool_classes_by_scheme['https'].ConnectionCls = VerifiedHTTPSConnectionWithHeaders

        return manager

def get(url, proxies, verify=True):
    with Session() as s:
        s.mount('https://', ProxyConnectWithHeadersHTTPSAdapter())
        s.proxies = proxies
        s.verify = verify
        response = s.get(url)

    return response


if __name__ == '__main__':
    logging.basicConfig()
    logging.getLogger().setLevel(logging.DEBUG)
    requests_log = logging.getLogger("requests.packages.urllib3")
    requests_log.setLevel(logging.DEBUG)
    requests_log.propagate = True

    proxies = { 'https': 'https://some_proxy:443' }

    url = 'https://www.google.com'
    r = get(url, proxies, verify=False)
    print(r.status_code)

from http.HTTPStatus import OK

This did not work for me on Python3.6.7; I changed to:

from http import HTTPStatus
OK = HTTPStatus.OK

See:

Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

from http.HTTPStatus import OK
Traceback (most recent call last):
File "", line 1, in
ModuleNotFoundError: No module named 'http.HTTPStatus'

Or:

Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

import http.HTTPStatus
Traceback (most recent call last):
File "", line 1, in
ModuleNotFoundError: No module named 'http.HTTPStatus'