Taking a file or pipe as input

Question

I wish to create a python program that is used from the command line.
It has one positional argument that is a file name.
I want to also be able to pass it input from a pipe

I want to create my python program so that it behaves the same here:

my_program.py input_data.txt

as it does here

cat input_data.txt | my_program.py

I have built programs that do one or the other, but I'm not sure how to conveniently handle both.
I usually use argparse to do this stuff so I was hoping there is an argparse based solution but if not that's fine.

Your program is supposed to read the contents of the csv file correct? — anarchy, Oct 11 '22 at 12:31
Seriously, don't bother. It's extra work that you do not need to do. Instead of `my_program.py input_data.txt`, you let the shell do the work and write `my_program.py < input_data.txt`. It's a better mind set anyway. — William Pursell, Oct 11 '22 at 12:40
As an aside, that's a [useless use of `cat`](https://stackoverflow.com/questions/11710552/useless-use-of-cat)) — tripleee, Oct 11 '22 at 12:56

score 1 · Accepted Answer · answered Oct 11 '22 at 12:45

1

You really shouldn't do this. Get in the habit of redirecting input, and life becomes much better. If you do want to do this, just add a few lines at the start:

if len(sys.argv) > 1:
    in_file = open(sys.argv[1], 'r')
else:
    in_file = sys.stdin

answered Oct 11 '22 at 12:45

William Pursell

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anarchy · Answer 2 · 2022-10-11T12:27:06.870

Make my_program.py read the name of the file in the argument.

If there is an argument, it will do the standard csv reader. But if it is piped a program, it will read the standard input from the pipe.

#! /usr/bin/python

import sys
import csv

arg = sys.argv[1] #this saves the filename to arg from the command line

if len(sys.argv) > 1: #if given an argument, run this
    with open(arg, newline='') as f:
        reader = csv.reader(f)
        for row in reader:
            print(row)

else: #if not given an argument, run this
    for line in sys.stdin:
        sys.stdout.write(line)

        with open(line, newline='') as f:
            reader = csv.reader(f)
            for row in reader:
                print(row)

You can edit the contents of the csv reader below depending on what your file looks like.

Taking a file or pipe as input

2 Answers2