#! /usr/bin/env python3

# Cutting Fields From A CSV File
# Programming Praxis 2021-12-07
# https://programmingpraxis.com/2021/12/07/cutting-fields-from-a-csv-file/

import argparse
import csv
import sys

from pathlib import Path

def parse_args():
    parser = argparse.ArgumentParser(prefix_chars=r'/-@',
        description='cut fields from a csv file by name')
    parser.add_argument('input_file', type=Path,
        help='input CSV file to be processed',
        metavar='INFILE.CSV')
    parser.add_argument('-o', '--output-file', metavar='OUTFILE.CSV',
        default=None, type=Path,
        help='output CSV file to be processed - stdout used if not specified')
    parser.add_argument('-f', '--fields', nargs = '+', metavar='FIELD',
        default=None, help='fields to cut from input file')
    parser.add_argument('--encoding', default='utf-8',
        help='encoding to read/write files, default UTF-8')
           
    args = parser.parse_args()
    return args

def read_csv_data(path, encoding):
    if path.stat().st_size == 0:
        sys.exit(f'{path} contains no data')
    with path.open('r', encoding=encoding, newline='') as csv_obj:
        reader = csv.reader(csv_obj)
        data = [row for row in reader]
    return data

def output_csv_data(data, path, encoding):
    if path is not None:
        with path.open('w', encoding=encoding, newline='') as csv_obj:
            writer = csv.writer(csv_obj)
            writer.writerows(data)
    else:
        writer = csv.writer(sys.stdout)
        writer.writerows(data)

def cut_fields(data, fields):
    if fields is None:
        return data
    headers = data[0]
    header_dict = {head: i for i, head in enumerate(headers)}
    wanted = [header_dict[field] for field in fields]
    out_data = [[row[i] for i in wanted] for row in data]
    return out_data

if __name__ == '__main__':
    args = parse_args()
    data = read_csv_data(args.input_file, args.encoding)
    output = cut_fields(data, args.fields)
    output_csv_data(output, args.output_file, args.encoding)