Python - Cómo llamo un atributo desde un main (if __name__ == '__main__')?

import sys

from PyQt5 import QtWidgets

from PyQt5.QtWidgets import QDialog, QApplication, QMainWindow

from PyQt5.uic import loadUi

from datetime import datetime

from sqlalchemy.ext.declarative import declarative_base

from sqlalchemy import create_engine

from sqlalchemy.orm import sessionmaker

from sqlalchemy import Column, Integer, String, DateTime

from sqlalchemy.sql.expression import null

engine = create_engine('postgresql://postgres:649785@localhost/prueba')

datos = declarative_base()

class ingresar(QMainWindow):

    def __init__(self):

        super(ingresar, self).__init__()

        loadUi("ingreso.ui", self)

        self.botonIngresar.clicked.connect(ingresarFuncion.db)

class ingresarFuncion(datos):

    __tablename__ = "users"

    id = Column(Integer(), primary_key=True)

    username = Column(String(50), nullable=False, unique=True)

    password_db = Column(String(50), nullable=False, unique=True)

    created_at = Column(DateTime(), default=datetime.now())

    def db(self):

        usuario = self.usuario.text()

        password = self.password.text()

        return usuario, password

Session = sessionmaker(engine)

session = Session()

if __name__ == '__main__':

    datos.metadata.drop_all(engine)

    datos.metadata.create_all(engine)

    user1= ingresarFuncion(username = "user", password_db = "user")

    session.add(user1)

    session.commit()

    users = session.query(ingresarFuncion).filter(

        ingresarFuncion.username == usuario

    ).filter(

       ingresarFuncion.password_db == password

    )

    if users:

        print("Existe")

    else:

        print("No existe")

app=QApplication(sys.argv)

mainwindow=ingresar()

widget =QtWidgets.QStackedWidget()

widget.addWidget(mainwindow)

widget.setFixedWidth(1200)

widget.setFixedHeight(600)

widget.show()

app.exec_()

ingresarFuncion.username == usuario