import os
from contextlib import contextmanager
from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker


class DBConfig:
    """Database configuration."""

    DB_NAME = os.environ['PSQL_DB_NAME']
    DB_HOST = os.environ['PSQL_DB_HOST']
    DB_PORT = int(os.environ.get('PSQL_DB_PORT', 5432))
    DB_USER = os.environ['PSQL_DB_USER']
    DB_PASS = os.environ['PSQL_DB_PASS']
    DB_POOL_RECYCLE = int(os.environ.get('PSQL_DB_POOL_RECYCLE', 7200))
    DB_POOL_SIZE = int(os.environ.get('PSQL_DB_POOL_SIZE', 1))


_config = DBConfig()
engine = create_engine(
    'postgresql+psycopg2://{user}:{password}@{host}:{port}/{database_name}'.format(
        user=_config.DB_USER,
        password=_config.DB_PASS,
        host=_config.DB_HOST,
        port=_config.DB_PORT,
        database_name=_config.DB_NAME),
    pool_recycle=_config.DB_POOL_RECYCLE, pool_size=_config.DB_POOL_SIZE
)

_session_factory = scoped_session(sessionmaker(bind=engine))
_session = _session_factory()


@contextmanager
def session_scope():
    """Provide a transactional scope around a series of operations.
    """
    try:
        yield _session
        _session.commit()
    except:
        _session.rollback()
        raise
    finally:
        _session.close()