set_to_public = await self.__TelegramClient(
                    UpdateUsernameRequest(channel=InputChannel(channel_id,channel_access_hash),username=username)
                )

api_id = ""
api_hash = ""
phone_number = ""
from telethon import TelegramClient, sync
from telethon.tl.functions.messages import GetHistoryRequest 

client = TelegramClient('session_name',api_id, api_hash)
client.connect()
if not client.is_user_authorized():
    client.send_code_request(phone_number)
    me = client.sign_in(phone_number, input('Enter code: '))
  
chat_username='helpers'
channel_entity=client.get_entity(chat_username)

posts = client(GetHistoryRequest(
    peer=channel_entity,
    limit=1000,
    offset_date=None,
    offset_id=0,
    max_id=0,
    min_id=0,
    add_offset=0,
    hash=0))
  
print("all ",len(posts.messages))

print(posts.messages[10].raw_text)