Detect STDIN file and prevent user input in Python

Question

I want to write a command-line Python program that can be called in a Windows cmd.exe prompt using the STDIN syntax and to print help text to STDOUT if an input file is not provided.

The STDIN syntax is different from argument syntax, and is necessary to be a drop-in replacement solution:

my_program.py < input.txt

Here's what I have so far:

import sys

# Define stdout with \n newline character instead of the \r\n default
stdout = open(sys.__stdout__.fileno(), 
              mode=sys.__stdout__.mode, 
              buffering=1, 
              encoding=sys.__stdout__.encoding, 
              errors=sys.__stdout__.errors, 
              newline='\n', 
              closefd=False)

def main(args):
    lines = ''.join([line for line in sys.stdin.readlines()])
    lines = lines.replace( '\r\n', '\n' ).replace( '\t', '    ' )    
    stdout.write(lines)

if __name__=='__main__':
    main(sys.argv)

I cannot figure out how to detect if a file was provided to STDIN and prevent prompting for user input if it wasn't. sys.argv doesn't contain STDIN. I could wrap it in a thread with a timer and wait for some file access upper limit time and decide that a file probably wasn't provided, but I wanted to see if there's a better way. I searched in SO for this question, but was unable to find an answer that avoids a timer.

Answer 1

test.py:

import sys

if sys.__stdin__.isatty():
    print("stdin from console")
else:
    print("stdin not from console")

execution:

> test.py
stdin from console
> test.py <input.txt
stdin not from console