Flexible UNIX command line interface with Python

Question

I was wondering how to create a flexible CLI interface with Python. So far I have come up with the following:

$ cat cat.py
#!/usr/bin/env python

from sys import stdin
from fileinput import input
from argparse import ArgumentParser, FileType

def main(args):

   for line in input():
      print line.strip()

if __name__ == "__main__":
   parser = ArgumentParser()
   parser.add_argument('FILE', nargs='?', type=FileType('r'), default=stdin)
   main(parser.parse_args())

This handles both stdin and file input:

$ echo 'stdin test' | ./cat.py
stdin test

$ ./cat.py file
file test

The problem is it doesn't handle multiple input or no input the way I would like:

$ ./cat.py file file
usage: cat.py [-h] [FILE]
cat.py: error: unrecognized arguments: file

$ ./cat.py

For multiple inputs it should cat the file multiple times and for no input input should ideally have same the behaviour as -h:

$ ./cat.py -h
usage: cat.py [-h] [FILE]

positional arguments:
  FILE

optional arguments:
  -h, --help  show this help message and exit

Any ideas on creating a flexible CLI interface with Python?

Why are you using both fileinput and argparse? input() does: "This iterates over the lines of all files listed in sys.argv[1:], defaulting to sys.stdin if the list is empty." Do you realize that argparse FileType opens the files that you name? As written you don't do anything with those opened files. — hpaulj
– hpaulj, Commented Oct 5, 2013 at 18:29
@hpaulj you are right I should pass the files to fileinput see my answer. — Chris Seymour
– Chris Seymour, Commented Oct 9, 2013 at 19:59

Martijn Pieters · Accepted Answer · 2013-10-05 12:57:24Z

4

Use nargs='*' to allow for 0 or more arguments:

if __name__ == "__main__":
    parser = ArgumentParser()
    parser.add_argument('FILE', nargs='*', type=FileType('r'), default=stdin)
    main(parser.parse_args())

The help output now is:

$ bin/python cat.py -h
usage: cat.py [-h] [FILE [FILE ...]]

positional arguments:
  FILE

optional arguments:
  -h, --help  show this help message and exit

and when no arguments are given, stdout is used.

If you want to require at least one FILE argument, use nargs='+' instead, but then the default is ignored, so you may as well drop that:

if __name__ == "__main__":
    parser = ArgumentParser()
    parser.add_argument('FILE', nargs='+', type=FileType('r'))
    main(parser.parse_args())

Now not specifying a command-line argument gives:

$ bin/python cat.py
usage: cat.py [-h] FILE [FILE ...]
cat.py: error: too few arguments

You can always specify stdin still by passing in - as an argument:

$ echo 'hello world!' | bin/python cat.py -
hello world!

edited Oct 5, 2013 at 12:57

answered Oct 5, 2013 at 12:51

Martijn Pieters

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3 Comments

Chris Seymour Over a year ago

This gets me 90% of the way there +1. Just handling stdin without - would be perfect.

Martijn Pieters Over a year ago

@sudo_O: That's handled by the nargs='*' and no arguments case; that's how defaults work. How did you envision stdin being handled in the nargs='+' case? You are then specifically stating there needs to be at least one argument.

Chris Seymour Over a year ago

Thanks gave me a push in the right direction, I figured it out in the end

Chris Seymour · Accepted Answer · 2013-10-09 20:22:12Z

2

A pretty good CLI interface the handles file input, standard input, no input, file output and inplace editing:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

def main(args, help):
    '''
    Simple line numbering program to demonstrate CLI interface
    '''

    if not (select.select([sys.stdin,],[],[],0.0)[0] or args.files):
        help()
        return

    if args.output and args.output != '-':
        sys.stdout = open(args.output, 'w')

    try:
        for i, line in enumerate(fileinput.input(args.files, inplace=args.inplace)):
            print i + 1, line.strip()
    except IOError:
        sys.stderr.write("%s: No such file %s\n" % 
                        (os.path.basename(__file__), fileinput.filename()))

if __name__ == "__main__":
    import os, sys, select, argparse, fileinput
    parser = argparse.ArgumentParser()
    parser.add_argument('files', nargs='*', help='input files')
    group = parser.add_mutually_exclusive_group()
    group.add_argument('-i', '--inplace', action='store_true', 
                       help='modify files inplace')
    group.add_argument('-o', '--output', 
                       help='output file. The default is stdout')
    main(parser.parse_args(), parser.print_help)

The code simply emulates nl and numbers the lines but should serve as a good skeleton for many applications.

edited Oct 9, 2013 at 20:22

answered Oct 9, 2013 at 19:58

Chris Seymour

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1 Comment

Martijn Pieters Over a year ago

Right, so with no arguments and no stdin pipe attached you show help, otherwise you read from stdin?

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