from sqlalchemy.dialects.postgresql import UUID
from sqlalchemy import Column, Integer, create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

from uuid import uuid4


engine = create_engine(r'postgresql://postgres:postgres@localhost/test')
Base = declarative_base()

class Test(Base):
    __tablename__ = 'test'
    id = Column(Integer, primary_key=True)
    uuid = Column(UUID, nullable=False, default=uuid4)

Base.metadata.create_all(engine)

t = Test()
Session = sessionmaker(bind=engine)
sess = Session()
sess.add(t)
sess.commit()

# throws error