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?
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!
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.
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