Read from stdin or files in Python (combining argparse and fileinput)

argparse_fileinput_demo.py

import argpase
import fileinput

if __name__ == '__main__':
    parser = ArgumentParser()
    parser.add_argument('--dummy', help='dummy argument')
    parser.add_argument('files', metavar='FILE', nargs='*', help='files to read, if empty, stdin is used')
    args = parser.parse_args()

    # If you would call fileinput.input() without files it would try to process all arguments.
    # We pass '-' as only file when argparse got no files which will cause fileinput to read from stdin
    for line in fileinput.input(files=args.files if len(args.files) > 0 else ('-', )):
        print(line)

In the special case where only 1 file (or none for stdin) is allowed, use: nargs='?', default=[].

The default= is generally required or the default will be None, which will cause fileinput to think it needs to read from sys.argv. This is a problem if you have options on the command line as it will then try to open them as files, and you'll get errors like No such file or directory: '-a'.

Passing an empty array (which is what nargs='*' does when no files are specified) avoids this.

Thank you, both this and @drmull's comment were very helpful