def _engine_builder(con):
    """
    Returns a SQLAlchemy engine from a URI (if con is a string)
    else it just return con without modifying it.
    """
    global _SQLALCHEMY_INSTALLED
    if isinstance(con, str):
        try:
            import sqlalchemy
        except ImportError:
            _SQLALCHEMY_INSTALLED = False
        else:
            con = sqlalchemy.create_engine(con)
            return con
    return con
