How to use the h11._headers.get_comma_header function in h11

def _process_event(self, role, event):
        # First, pass the event through the state machine to make sure it
        # succeeds.
        old_states = dict(self._cstate.states)
        if role is CLIENT and type(event) is Request:
            if event.method == b"CONNECT":
                self._cstate.process_client_switch_proposal(_SWITCH_CONNECT)
            if get_comma_header(event.headers, b"upgrade"):
                self._cstate.process_client_switch_proposal(_SWITCH_UPGRADE)
        server_switch_event = None
        if role is SERVER:
            server_switch_event = self._server_switch_event(event)
        self._cstate.process_event(role, type(event), server_switch_event)

        # Then perform the updates triggered by it.

        # self._request_method
        if type(event) is Request:
            self._request_method = event.method

        # self.their_http_version
        if role is self.their_role and type(event) in (
            Request,
            Response,

or (request_method == b"CONNECT" and 200 <= event.status_code < 300)
        ):
            return ("content-length", (0,))
        # Section 3.3.3 also lists another case -- responses with status_code
        # < 200. For us these are InformationalResponses, not Responses, so
        # they can't get into this function in the first place.
        assert event.status_code >= 200

    # Step 2: check for Transfer-Encoding (T-E beats C-L):
    transfer_encodings = get_comma_header(event.headers, b"transfer-encoding")
    if transfer_encodings:
        assert transfer_encodings == [b"chunked"]
        return ("chunked", ())

    # Step 3: check for Content-Length
    content_lengths = get_comma_header(event.headers, b"content-length")
    if content_lengths:
        return ("content-length", (int(content_lengths[0]),))

    # Step 4: no applicable headers; fallback/default depends on type
    if type(event) is Request:
        return ("content-length", (0,))
    else:
        return ("http/1.0", ())

def _keep_alive(event):
    connection = get_comma_header(event.headers, b"connection")
    if b"close" in connection:
        return False
    if getattr(event, "http_version", b"1.1") < b"1.1":
        return False
    return True

# error (their_http_version is None), so we assume the worst;
                # or else we did get a valid HTTP/1.0 request, so we know that
                # they don't understand chunked encoding.
                set_comma_header(headers, b"transfer-encoding", [])
                # This is actually redundant ATM, since currently we
                # unconditionally disable keep-alive when talking to HTTP/1.0
                # peers. But let's be defensive just in case we add
                # Connection: keep-alive support later:
                if self._request_method != b"HEAD":
                    need_close = True
            else:
                set_comma_header(headers, b"transfer-encoding", ["chunked"])

        if not self._cstate.keep_alive or need_close:
            # Make sure Connection: close is set
            connection = set(get_comma_header(headers, b"connection"))
            connection.discard(b"keep-alive")
            connection.add(b"close")
            set_comma_header(headers, b"connection", sorted(connection))

        response.headers = headers

# Step 1: some responses always have an empty body, regardless of what the
    # headers say.
    if type(event) is Response:
        if (
            event.status_code in (204, 304)
            or request_method == b"HEAD"
            or (request_method == b"CONNECT" and 200 <= event.status_code < 300)
        ):
            return ("content-length", (0,))
        # Section 3.3.3 also lists another case -- responses with status_code
        # < 200. For us these are InformationalResponses, not Responses, so
        # they can't get into this function in the first place.
        assert event.status_code >= 200

    # Step 2: check for Transfer-Encoding (T-E beats C-L):
    transfer_encodings = get_comma_header(event.headers, b"transfer-encoding")
    if transfer_encodings:
        assert transfer_encodings == [b"chunked"]
        return ("chunked", ())

    # Step 3: check for Content-Length
    content_lengths = get_comma_header(event.headers, b"content-length")
    if content_lengths:
        return ("content-length", (int(content_lengths[0]),))

    # Step 4: no applicable headers; fallback/default depends on type
    if type(event) is Request:
        return ("content-length", (0,))
    else:
        return ("http/1.0", ())