Optional stdin in Python with argparse

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I found the very useful syntax

parser.add_argument('-i', '--input-file', type=argparse.FileType('r'), default="-")

for specifying an input file or using stdin—both of which I want in my program. However, the input file is not always required. If I’m not using -i or redirecting input with one of

$ someprog | my_python_prog
$ my_python_prog < inputfile

I don’t want my Python program to wait for input. I want it to just move along and use default values.

The standard library documentation for argparse suggests this solution to allow optional input/output files:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('infile', nargs="?", type=argparse.FileType('r'),
...                     default=sys.stdin)
>>> parser.add_argument('outfile', nargs="?", type=argparse.FileType('w'),
...                     default=sys.stdout)
>>> parser.parse_args(['input.txt', 'output.txt'])
Namespace(infile=<_io.TextIOWrapper name="input.txt" encoding='UTF-8'>,
          outfile=<_io.TextIOWrapper name="output.txt" encoding='UTF-8'>)
>>> parser.parse_args([])
Namespace(infile=<_io.TextIOWrapper name="<stdin>" encoding='UTF-8'>,
          outfile=<_io.TextIOWrapper name="<stdout>" encoding='UTF-8'>)

Use isatty to detect whether your program is in an interactive session or reading from a file:

if not sys.stdin.isatty(): # Not an interactive device.
  # ... read from stdin

However, for the sake of consistency and reproducability, consider following the norm and reading from stdin if the filename is -. You may want to consider to let the fileinput module handle that.

Building on top of the answer regarding TTY detection, to answer the question explicitly:

import sys
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('-i', '--input-file', type=argparse.FileType('r'), default=(None if sys.stdin.isatty() else sys.stdin))


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