Keyset Pagination Cursor

from dataclasses import dataclass
import base64
import json
from typing import Optional

@dataclass(frozen=True)
class Cursor:
    created_at: str
    row_id: int

def encode(cursor: Optional[Cursor]) -> Optional[str]:
    if cursor is None:
        return None
    raw = json.dumps({'c': cursor.created_at, 'i': cursor.row_id}).encode('utf-8')
    return base64.urlsafe_b64encode(raw).decode('ascii')

def decode(token: Optional[str]) -> Optional[Cursor]:
    if not token:
        return None
    try:
        data = json.loads(base64.urlsafe_b64decode(token.encode('ascii')))
        return Cursor(created_at=str(data['c']), row_id=int(data['i']))
    except (ValueError, KeyError, TypeError):
        raise ValueError('invalid pagination cursor')

def fetch_page(conn, cursor: Optional[Cursor], limit: int = 20):
    if cursor is None:
        rows = conn.execute(
            'SELECT id, created_at FROM items ORDER BY created_at, id LIMIT ?', (limit + 1,)).fetchall()
    else:
        rows = conn.execute(
            'SELECT id, created_at FROM items WHERE (created_at, id) > (?, ?) '
            'ORDER BY created_at, id LIMIT ?', (cursor.created_at, cursor.row_id, limit + 1)).fetchall()
    has_more = len(rows) > limit
    page = rows[:limit]
    next_cursor = Cursor(page[-1][1], page[-1][0]) if has_more and page else None
    return page, encode(next_cursor)