def str_slice(df, column_name: str, start: int = 0, stop: int = -1):
    """Slice a column of strings by indexes

    Parameters
    -----------
    df: pd.Dataframe
        Input dataframe to be modified
    column_name: str
        Name of the column to be operated on
    start: int, optional, default to 0
        Staring index for string slicing
    stop: int, optional, default to -1
        Ending index for string slicing

    Returns
    --------
    df: pd.Dataframe

    """
    df[column_name] = df[column_name].str[start:stop]
    return df