Object Pool

# Author: Filip Sawicki


# from sqlalchemy import create_engine

USER = 'FILIP'
PASS = 'ADMIN'
SERVER = 'localhost'


class Singleton():
    def __new__(cls, *args, **kwds):
        self = "__self__"
        if not hasattr(cls, self):
            instance = object.__new__(cls)
            instance.init(*args, **kwds)
            setattr(cls, self, instance)
        else:
            raise Exception("This is a singleton class.")
        return getattr(cls, self)

    def init(self, *args, **kwds):
        pass


class DataBase():
    def __init__(self):
        self.connection = None
        self.engine = None
        self.open = False

    def __del__(self):
        if self.open:
            # self.disconnect()
            self.open = False
        del self

    def connect(self, driver):
        self.engine = create_engine(driver)
        # self.connection = self.engine.connect()
        self.open = True

    def disconnect(self):
        # self.connection.close()
        self.open = False

    def execute(self, request):
        print('Executing -> {}'.format(request))
        # return self.connection.execute(request)


class DataBasePool(Singleton):
    def __init__(self, size):
        self.max_pool_size = size
        self.db_pool = [DataBase() for _ in range(0, self.max_pool_size)]

    def acquire(self):
        return self.db_pool.pop()

    def release(self, db):
        if len(self.db_pool) < self.max_pool_size:
            self.db_pool.append(db)
        else:
            print('Limit Reached')

    def cleanup(self):
        for index, db in enumerate(self.db_pool):
            del db
            self.db_pool[index] = DataBase()


def create_engine(driver):
    print('Creating connection...')
    # return sqlalchemy.create_engine(driver)


def main():
    db_pool = DataBasePool(2)
    # db_pool2 = DataBasePool(3)

    db1 = db_pool.acquire()
    db1.connect('mysql://{}:{}@{}/db'.format(USER, PASS, SERVER))
    result = db1.execute('SELECT username FROM user')

    db_pool.release(db1)
    db_pool.cleanup()


if __name__ == "__main__":
    main()