Read from stdin or input file with argparse

Question

I'd like to use argparse to read from either stdin or an input file. In other words:

• If an input file is given, read that.
• If not, read from stdin only if it's not the terminal. (i.e. a file is being piped in)
• If neither of these criteria are satisfied, signal to argparse that the inputs aren't correct.

I'm asking for behavior similar to what's described in this question, but I want argparse to recognize no file as a failed input.

Answer 1

I would recommend just settings nargs='?' and then handling the case of a Nonetype separately. According to the official documentation, "FileType objects understand the pseudo-argument '-' and automatically convert this into sys.stdin for readable FileType objects and sys.stdout for writable FileType objects". So just give it a dash if you want stdin.

Example

import argparse
import sys
parser = argparse.ArgumentParser()
parser.add_argument('inputfile', nargs='?', type=argparse.FileType('r'))
if not inputfile:
    sys.exit("Please provide an input file, or pipe it via stdin")

Answer 2

Using the information from the question you linked to, what about using sys.stdin.isatty() to check if the instance your program is being run is part of a pipeline, if not, read from input file, otherwise read from stdin. If the input file does not exist or stdin is empty throw an error.

Hope that helped.