How to use the multidict.istr function in multidict

CONTENT_ENCODING = istr('Content-Encoding')
CONTENT_LANGUAGE = istr('Content-Language')
CONTENT_LENGTH = istr('Content-Length')
CONTENT_LOCATION = istr('Content-Location')
CONTENT_MD5 = istr('Content-MD5')
CONTENT_RANGE = istr('Content-Range')
CONTENT_TRANSFER_ENCODING = istr('Content-Transfer-Encoding')
CONTENT_TYPE = istr('Content-Type')
COOKIE = istr('Cookie')
DATE = istr('Date')
DESTINATION = istr('Destination')
DIGEST = istr('Digest')
ETAG = istr('Etag')
EXPECT = istr('Expect')
EXPIRES = istr('Expires')
FORWARDED = istr('Forwarded')
FROM = istr('From')
HOST = istr('Host')
IF_MATCH = istr('If-Match')
IF_MODIFIED_SINCE = istr('If-Modified-Since')
IF_NONE_MATCH = istr('If-None-Match')
IF_RANGE = istr('If-Range')
IF_UNMODIFIED_SINCE = istr('If-Unmodified-Since')
KEEP_ALIVE = istr('Keep-Alive')
LAST_EVENT_ID = istr('Last-Event-ID')
LAST_MODIFIED = istr('Last-Modified')
LINK = istr('Link')
LOCATION = istr('Location')
MAX_FORWARDS = istr('Max-Forwards')
ORIGIN = istr('Origin')
PRAGMA = istr('Pragma')
PROXY_AUTHENTICATE = istr('Proxy-Authenticate')

CONTENT_DISPOSITION = istr('Content-Disposition')
CONTENT_ENCODING = istr('Content-Encoding')
CONTENT_LANGUAGE = istr('Content-Language')
CONTENT_LENGTH = istr('Content-Length')
CONTENT_LOCATION = istr('Content-Location')
CONTENT_MD5 = istr('Content-MD5')
CONTENT_RANGE = istr('Content-Range')
CONTENT_TRANSFER_ENCODING = istr('Content-Transfer-Encoding')
CONTENT_TYPE = istr('Content-Type')
COOKIE = istr('Cookie')
DATE = istr('Date')
DESTINATION = istr('Destination')
DIGEST = istr('Digest')
ETAG = istr('Etag')
EXPECT = istr('Expect')
EXPIRES = istr('Expires')
FORWARDED = istr('Forwarded')
FROM = istr('From')
HOST = istr('Host')
IF_MATCH = istr('If-Match')
IF_MODIFIED_SINCE = istr('If-Modified-Since')
IF_NONE_MATCH = istr('If-None-Match')
IF_RANGE = istr('If-Range')
IF_UNMODIFIED_SINCE = istr('If-Unmodified-Since')
KEEP_ALIVE = istr('Keep-Alive')
LAST_EVENT_ID = istr('Last-Event-ID')
LAST_MODIFIED = istr('Last-Modified')
LINK = istr('Link')
LOCATION = istr('Location')
MAX_FORWARDS = istr('Max-Forwards')
ORIGIN = istr('Origin')
PRAGMA = istr('Pragma')

SERVER = istr('Server')
SET_COOKIE = istr('Set-Cookie')
TE = istr('TE')
TRAILER = istr('Trailer')
TRANSFER_ENCODING = istr('Transfer-Encoding')
UPGRADE = istr('Upgrade')
WEBSOCKET = istr('WebSocket')
URI = istr('URI')
USER_AGENT = istr('User-Agent')
VARY = istr('Vary')
VIA = istr('Via')
WANT_DIGEST = istr('Want-Digest')
WARNING = istr('Warning')
WWW_AUTHENTICATE = istr('WWW-Authenticate')
X_FORWARDED_FOR = istr('X-Forwarded-For')
X_FORWARDED_HOST = istr('X-Forwarded-Host')
X_FORWARDED_PROTO = istr('X-Forwarded-Proto')

from multidict import istr, CIMultiDict


CHARSET = 'ISO-8859-1'
CRLF = b'\r\n'
HEAD = 'HEAD'
MAX_CHUNK_SIZE = 65536
TLS_SCHEMES = frozenset(('https', 'wss'))
NO_CONTENT_CODES = frozenset((204, 304))
NO_BODY_VERBS = frozenset((HEAD,))
URL_SCHEME = os.environ.get('wsgi.url_scheme', 'http')
OS_SCRIPT_NAME = os.environ.get("SCRIPT_NAME", "")
PULSAR_CACHE = 'pulsar.cache'
CONNECTION = istr('Connection')
CONTENT_LENGTH = istr('Content-Length')
CONTENT_TYPE = istr('Content-Type')
DATE = istr('Date')
HOST = istr('Host')
KEEP_ALIVE = istr('Keep-Alive')
LOCATION = istr('Location')
PROXY_AUTHENTICATE = istr('Proxy-Authenticate')
PROXY_AUTHORIZATION = istr('Proxy-Authorization')
SCRIPT_NAME = istr("Script_Name")
SERVER = istr('Server')
SET_COOKIE = istr('Set-Cookie')
TE = istr('Te')
TRAILERS = istr('Trailers')
TRANSFER_ENCODING = istr('Transfer-Encoding')
UPGRADE = istr('Upgrade')
X_FORWARDED_FOR = istr('X-Forwarded-For')
EXPECT = istr('Expect')
HOP_HEADERS = frozenset((

EXPIRES = istr('Expires')
FORWARDED = istr('Forwarded')
FROM = istr('From')
HOST = istr('Host')
IF_MATCH = istr('If-Match')
IF_MODIFIED_SINCE = istr('If-Modified-Since')
IF_NONE_MATCH = istr('If-None-Match')
IF_RANGE = istr('If-Range')
IF_UNMODIFIED_SINCE = istr('If-Unmodified-Since')
KEEP_ALIVE = istr('Keep-Alive')
LAST_EVENT_ID = istr('Last-Event-ID')
LAST_MODIFIED = istr('Last-Modified')
LINK = istr('Link')
LOCATION = istr('Location')
MAX_FORWARDS = istr('Max-Forwards')
ORIGIN = istr('Origin')
PRAGMA = istr('Pragma')
PROXY_AUTHENTICATE = istr('Proxy-Authenticate')
PROXY_AUTHORIZATION = istr('Proxy-Authorization')
RANGE = istr('Range')
REFERER = istr('Referer')
RETRY_AFTER = istr('Retry-After')
SEC_WEBSOCKET_ACCEPT = istr('Sec-WebSocket-Accept')
SEC_WEBSOCKET_VERSION = istr('Sec-WebSocket-Version')
SEC_WEBSOCKET_PROTOCOL = istr('Sec-WebSocket-Protocol')
SEC_WEBSOCKET_EXTENSIONS = istr('Sec-WebSocket-Extensions')
SEC_WEBSOCKET_KEY = istr('Sec-WebSocket-Key')
SEC_WEBSOCKET_KEY1 = istr('Sec-WebSocket-Key1')
SERVER = istr('Server')
SET_COOKIE = istr('Set-Cookie')
TE = istr('TE')
TRAILER = istr('Trailer')

from multidict import istr

X_FORWARDED_PROTO = istr("X-Forwarded-Proto")
X_FORWARDED_HOST = istr("X-Forwarded-Host")
X_FORWARDED_PORT = istr("X-Forwarded-Port")

def _init_header(
    request: web.Request, addon: str
) -> Union[CIMultiDict, Dict[str, str]]:
    """Create initial header."""
    headers = {}

    # filter flags
    for name, value in request.headers.items():
        if name in (hdrs.CONTENT_LENGTH, hdrs.CONTENT_ENCODING, istr(HEADER_TOKEN)):
            continue
        headers[name] = value

    # Update X-Forwarded-For
    forward_for = request.headers.get(hdrs.X_FORWARDED_FOR)
    connected_ip = ip_address(request.transport.get_extra_info("peername")[0])
    headers[hdrs.X_FORWARDED_FOR] = f"{forward_for}, {connected_ip!s}"

    return headers

ACCESS_CONTROL_REQUEST_METHOD = istr('Access-Control-Request-Method')
AGE = istr('Age')
ALLOW = istr('Allow')
AUTHORIZATION = istr('Authorization')
CACHE_CONTROL = istr('Cache-Control')
CONNECTION = istr('Connection')
CONTENT_DISPOSITION = istr('Content-Disposition')
CONTENT_ENCODING = istr('Content-Encoding')
CONTENT_LANGUAGE = istr('Content-Language')
CONTENT_LENGTH = istr('Content-Length')
CONTENT_LOCATION = istr('Content-Location')
CONTENT_MD5 = istr('Content-MD5')
CONTENT_RANGE = istr('Content-Range')
CONTENT_TRANSFER_ENCODING = istr('Content-Transfer-Encoding')
CONTENT_TYPE = istr('Content-Type')
COOKIE = istr('Cookie')
DATE = istr('Date')
DESTINATION = istr('Destination')
DIGEST = istr('Digest')
ETAG = istr('Etag')
EXPECT = istr('Expect')
EXPIRES = istr('Expires')
FORWARDED = istr('Forwarded')
FROM = istr('From')
HOST = istr('Host')
IF_MATCH = istr('If-Match')
IF_MODIFIED_SINCE = istr('If-Modified-Since')
IF_NONE_MATCH = istr('If-None-Match')
IF_RANGE = istr('If-Range')
IF_UNMODIFIED_SINCE = istr('If-Unmodified-Since')
KEEP_ALIVE = istr('Keep-Alive')
LAST_EVENT_ID = istr('Last-Event-ID')

CONTENT_LENGTH = istr('Content-Length')
CONTENT_TYPE = istr('Content-Type')
DATE = istr('Date')
HOST = istr('Host')
KEEP_ALIVE = istr('Keep-Alive')
LOCATION = istr('Location')
PROXY_AUTHENTICATE = istr('Proxy-Authenticate')
PROXY_AUTHORIZATION = istr('Proxy-Authorization')
SCRIPT_NAME = istr("Script_Name")
SERVER = istr('Server')
SET_COOKIE = istr('Set-Cookie')
TE = istr('Te')
TRAILERS = istr('Trailers')
TRANSFER_ENCODING = istr('Transfer-Encoding')
UPGRADE = istr('Upgrade')
X_FORWARDED_FOR = istr('X-Forwarded-For')
EXPECT = istr('Expect')
HOP_HEADERS = frozenset((
    CONNECTION, KEEP_ALIVE, PROXY_AUTHENTICATE,
    PROXY_AUTHORIZATION, TE, TRAILERS,
    TRANSFER_ENCODING, UPGRADE
))


class Headers:
    __slots__ = ()

    def X_FORWARDED_PROTOCOL(self, environ, value):
        if value == "ssl":
            environ['wsgi.url_scheme'] = 'https'

    def X_FORWARDED_PROTO(self, environ, value):

COOKIE = istr('Cookie')
DATE = istr('Date')
DESTINATION = istr('Destination')
DIGEST = istr('Digest')
ETAG = istr('Etag')
EXPECT = istr('Expect')
EXPIRES = istr('Expires')
FORWARDED = istr('Forwarded')
FROM = istr('From')
HOST = istr('Host')
IF_MATCH = istr('If-Match')
IF_MODIFIED_SINCE = istr('If-Modified-Since')
IF_NONE_MATCH = istr('If-None-Match')
IF_RANGE = istr('If-Range')
IF_UNMODIFIED_SINCE = istr('If-Unmodified-Since')
KEEP_ALIVE = istr('Keep-Alive')
LAST_EVENT_ID = istr('Last-Event-ID')
LAST_MODIFIED = istr('Last-Modified')
LINK = istr('Link')
LOCATION = istr('Location')
MAX_FORWARDS = istr('Max-Forwards')
ORIGIN = istr('Origin')
PRAGMA = istr('Pragma')
PROXY_AUTHENTICATE = istr('Proxy-Authenticate')
PROXY_AUTHORIZATION = istr('Proxy-Authorization')
RANGE = istr('Range')
REFERER = istr('Referer')
RETRY_AFTER = istr('Retry-After')
SEC_WEBSOCKET_ACCEPT = istr('Sec-WebSocket-Accept')
SEC_WEBSOCKET_VERSION = istr('Sec-WebSocket-Version')
SEC_WEBSOCKET_PROTOCOL = istr('Sec-WebSocket-Protocol')
SEC_WEBSOCKET_EXTENSIONS = istr('Sec-WebSocket-Extensions')