Let's Make Better Command Line Applications 

Notes, Examples, Slides, Feedback: http://daveops.com/pyohio2016 

Philosophy 

The Zen of Python >>> import this The Zen of Python, by Tim Peters Beautiful is better than ugly. Explicit is better than implicit. Simple is better than complex. Complex is better than complicated. Flat is better than nested. Sparse is better than dense. Readability counts. Special cases aren't special enough to break the rules. Although practicality beats purity. Errors should never pass silently. Unless explicitly silenced. In the face of ambiguity, refuse the temptation to guess. There should be one-- and preferably only one --obvious way to do it. Although that way may not be obvious at first unless you're Dutch. Now is better than never. Although never is often better than *right* now. If the implementation is hard to explain, it's a bad idea. If the implementation is easy to explain, it may be a good idea. Namespaces are one honking great idea -- let's do more of those! 

The Unix Philosophy Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface. 

Working Together POSIX 

Standard Streams Text terminal Keyboard Display Program ① stdin ② stdout ③ stderr Image by - Own work, Public Domain, ScotXW https://commons.wikimedia.org/w/index.php?curid=687268 

Redirection $ grep "Hello" * grep: example_directory: Is a directory example.txt:Hello, World! grep: protected-file.txt: Permission denied $ grep "Hello" * > matches.txt grep: example_directory: Is a directory grep: protected-file.txt: Permission denied $ cat matches.txt example.txt:Hello, World! $ grep "Hello" * > matches.txt 2> diagnostics.txt $ cat diagnostics.txt grep: example_directory: Is a directory grep: protected-file.txt: Permission denied 

Pipelines ls -l | grep example | less Image by - en:File:Pipeline.svg, Public Domain, TyIzaeL https://commons.wikimedia.org/w/index.php?curid=11524651 

Why CLI? 

Why Python? Stdlib + PyPI “Readability counts.” (The Zen of Python) Argument parsing 

Why not Python? 

A Good Python Command Line App ...'s design is informed by The Zen of Python and The Unix Philosophy. 

Arguments & Options example ARGUMENT --long-option or -l --help --verbose 

std(in|out|err) 

Exit code 

Ctrl-c / Signals 

Configuration Standard location (per-platorm) Overridable 

Config Specificity Default value Config value Env variable CLI option 

Colors! Can be helpful ...when used correctly & can be disabled 

Parsers stdlib PyPI sys.argv getopt optparse argparse Cement Click Cliff Clint Compago Docopt Plac ... 

Boilerplate 

#!/usr/bin/python #! #!/usr/bin/env python or for Python 3 only: #!/usr/bin/env python3 not: 

Execute or Import def cli(): pass # do stuff here if __name__ == '__main__': cli() 

Package Layout pyohio ├── __init__.py ├── cli.py └── pyohio.py 

Entry Points setup.py setup( … entry_points={ 'console_scripts':[ 'hello-world=pyohio.cli:hello', 'pyohio=pyohio.cli:cli', ], }, … ) 

Argparse 

hello_argparse.py import argparse def hello(): """ Argparse CLI to greet the provided NAME. """ parser = argparse.ArgumentParser() parser.add_argument('name', help="name of person to greet") parser.add_argument('-c', '--count', type=int, default=1, help="number of times to print the greeting") args = parser.parse_args() for i in xrange(args.count): print("Hello, {0}!".format(args.name)) 

hello-argparse --help $ hello-argparse --help usage: hello-argparse [-h] [-c COUNT] name positional arguments: name name of person to greet optional arguments: -h, --help show this help message and exit -c COUNT, --count COUNT number of times to print the greeting 

hello-argparse $ hello-argparse usage: hello-argparse [-h] [-c COUNT] name hello-argparse: error: too few arguments $ hello-argparse PyOhio Hello, PyOhio! $ hello-argparse --count 3 PyOhio Hello, PyOhio! Hello, PyOhio! Hello, PyOhio! 

Click 

First: Decorators def wrapper(func): # wraps a function in order to add additional behavior @wrapper def my_function(example_arg): # does stuff 

hello_click.py import click @click.command() @click.argument('name') @click.option('--count', '-c', default=1, help="Number of times to print the greeting") def hello(name, count): """ Click CLI to greet the provided NAME. """ for i in xrange(count): print("Hello, {0}!".format(name)) 

hello-click --help $ hello-click --help Usage: hello-click [OPTIONS] NAME Click CLI to greet the provided NAME. Options: -c, --count INTEGER Number of times to print the greeting --help Show this message and exit. 

hello-click $ hello-click Usage: hello-click [OPTIONS] NAME Error: Missing argument "name". $ hello-click PyOhio Hello, PyOhio! $ hello-click --count 3 PyOhio Hello, PyOhio! Hello, PyOhio! Hello, PyOhio! 

Docopt 

hello_docopt.py """Hello, Docopt. Usage: hello-docopt hello-docopt [--count=] hello-docopt -h | --help Options: -h --help Show this screen. -c --count= Times to print greeting [default: 1]. """ from docopt import docopt def cli(): args = docopt(__doc__) for i in xrange(int(args['--count'])): print("Hello, {0}!".format(args[''])) 

hello-docopt --help $ hello-docopt --help Hello, Docopt. Usage: hello-docopt hello-docopt [--count=] hello-docopt -h | --help Options: -h --help Show this screen. -c --count= Times to print greeting [default: 1]. 

hello-docopt $ hello-docopt Usage: hello-docopt hello-docopt [--count=] hello-docopt -h | --help $ hello-docopt PyOhio Hello, PyOhio! $ hello-docopt --count 3 PyOhio Hello, PyOhio! Hello, PyOhio! Hello, PyOhio! 

Let's make a better CLI app! Read from a file or stdin Write to a file or stdout Update to log or stderr Has an option or two Has colors! Does something 

Sample data Declaration of Independence [Adopted in Congress 4 July 1776] The Unanimous Declaration of the Thirteen United States of When, in the course of human events, it becomes necessary for dissolve the political bands which have connected them with an assume among the powers of the earth, the separate and equal s which the laws of nature and of nature's God entitle them, a d to the opinions of mankind requires that they should declare t which impel them to the separation. … 

cli_click.py import click import sys from pyohio2016 import counter @click.command() @click.argument('infile', type=click.File('r'), default='-') @click.argument('outfile', type=click.File('w'), default='-') @click.option('--log-file', '-l', type=click.File('w'), default=sys.stderr) @click.option('--verbose', '-v', is_flag=True) def cli(infile, outfile, log_file, verbose): """ Count char occurances in INFILE, output to OUTFILE. """ … 

cli_click.py (cont'd) … if verbose: click.echo("Infile: {0}".format(infile), file=log_file) click.echo("Outfile: {0}".format(outfile), file=log_file) text = infile.read() char_counts = counter.count_chars(text) output = "\n".join( ["{0} {1}".format(k, v) for k, v in char_counts]) click.echo("Counted characters...", file=log_file) click.secho(output, file=outfile, fg='green') if verbose: click.echo("Done.".format(outfile), file=log_file) 

pyohio-click --help Usage: pyohio-click [OPTIONS] [INFILE] [OUTFILE] Count char occurances in INFILE, output to OUTFILE. Options: -l, --log-file FILENAME -v, --verbose --help Show this message and exit. 

File input, stdout output $ pyohio-click data/declaration.txt Counted characters... 1706 e 875 t 640 o 522 n 493 a 479 s 477 i 454 r 429 … 

stdin input $ echo -n "test" | pyohio-click Counted characters... t 2 s 1 e 1 $ echo -n "test" | pyohio-click - results.txt Counted characters... $ cat results.txt t 2 s 1 e 1 

Verbose with log file $ echo -n "test" | pyohio-click -v --log-file log.txt - out.txt $ cat out.txt t 2 s 1 e 1 $ cat log.txt Infile: <_io.TextIOWrapper name='' encoding='utf-8'> Outfile: Counted characters... Done. 

Further Reading 

Packaging 

Cookiecutter 

Logging 

The Zen of Python + The Unix Philosophy = Better Python CLI 

Thank You! Notes, Examples, Slides, Feedback: Questions? [email protected] @tylerdave http://daveops.com/pyohio2016