from contextlib import contextmanager

from sqlalchemy import create_engine
from sqlalchemy.exc import SQLAlchemyError
from sqlalchemy.orm import scoped_session, sessionmaker


DB_URI = 'mysql+pymysql://{user}:{password}@{host}:{port}/{database_name}'
_engine = None


def get_engine(config):
    """Create mysql engine"""
    global _engine

    if not _engine:
        _engine = create_engine(DB_URI.format(
            user=config.DB_USER,
            password=config.DB_PASS,
            host=config.DB_HOST,
            port=config.DB_PORT,
            database_name=config.DB_NAME))

    return _engine


@contextmanager
def session_scope(config):
    """Provide a transactional scope around a series of operations."""
    engine = get_engine(config=config)
    Session = scoped_session(sessionmaker(bind=engine))
    session = Session()
    try:
        yield session
    except SQLAlchemyError:
        session.rollback()
        raise
    finally:
        session.close()