def colname_gen(df, col_name='unnamed_col'):
    """ Returns a column name that isn't in the specified DataFrame
    Parameters:
    df - DataFrame
        DataFrame to analyze
    col_name - string, default 'unnamed_col'
        Column name to use as the base value for the generated column name
    """
    if col_name not in df.keys():
        yield col_name
    id_number = 0
    while True:
        col_name = col_name + str(id_number)
        if col_name in df.keys():
            id_number += 1
        else:
            return col_name
