Invalid Semantics

from sqlalchemy import create_engine, Column, types
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
connection_string = "sqlite:///database.db"   # for SQLite, local file
db   = create_engine(connection_string)
base = declarative_base()
class Book(base):
    __tablename__ = 'books'
    id = Column(types.Integer, primary_key=True)
    author = Column(types.String(length=50))
    title = Column(types.String(length=120), nullable=False)
    available = Column(types.Boolean, nullable=False)
class BirthYear(base):
    __tablename__ = 'birth_years'
    author = Column(types.String(length=50), primary_key=True)
    birth_year = Column(types.Integer, nullable=False)