replace-stdin.py

import argparse
import csv
import sys

parser = argparse.ArgumentParser(description='Replace values on a column level in CSV')

parser.add_argument('--column', help='The column to replace in', required=True)
parser.add_argument('--search', help='The value to search for', required=True)
parser.add_argument('--replace', help='The value to replace it with', required=True)
parser.add_argument('data', help='The data being operated on', nargs='?',
                    type=argparse.FileType('r'), default=sys.stdin)

args = parser.parse_args()

csv_reader = csv.reader(args.data)
header = next(csv_reader)
print(','.join(header))
try:
    column_index = header.index(args.column)
except ValueError as e:
    raise Exception('Column', args.column, 'not in list.')

for row in csv_reader:
    if row[column_index] == args.search:
        row[column_index] = args.replace
    print(','.join(row))

data.csv:

First,Second,Third
a,b,c
x,y,z

cat data.csv | replace.py --column Second --search b --replace B
and
replace.py --column Second --search b --replace B data.csv
should both produce

First,Second,Third
a,B,c
x,y,z