optfunc

Parse command line options in Python using function introspection.

Post feedback here: http://simonwillison.net/2009/May/28/optfunc/

I can never remember how to use any of Python's regular command line parsing libraries.

optfunc uses introspection to make a Python function available as a command line utility. It's syntactic sugar around optparse from the standard library.

Here's what the API looks like so far:

import optfunc

def upper(filename, verbose = False):
    "Usage: %prog <file> [--verbose] - output file content in uppercase"
    s = open(filename).read()
    if verbose:
        print "Processing %s bytes..." % len(s)
    print s.upper()

if __name__ == '__main__':
    optfunc.run(upper)

And here's the resulting command-line interface:

$ ./demo.py --help
Usage: demo.py <file> [--verbose] - output file content in uppercase

Options:
  -h, --help     show this help message and exit
  -v, --verbose  
$ ./demo.py README.txt 
OPTFUNC
...
$ ./demo.py README.txt -v
Processing 2049 bytes...
OPTFUNC
...

If you don't mind relying on some stack inspecting magic, you can replace the name == 'main idiom with the following:

optfunc.main(upper)

If you like really short scripts, you can even use this function as a decorator:

@optfunc.main
def upper(filename):
    print open(filename).read().upper()

How arguments work

Non-keyword arguments are treated as required arguments - optfunc.run will throw an error if they number of arguments provided on the command line doesn't match the number expected by the function (unless @notstrict is used, see below).

Keyword arguments with defaults are treated as options. At the moment, only string and boolean arguments are supported. Other types are planned.

Consider the following:

def geocode(s, api_key='', geocoder='google', list_geocoders=False):

's' is a required argument. api_key, geocoder and list_geocoders are all options, with defaults provided. Since list_geocoders has a boolean as its default it will be treated slightly differently (in optparse terms, it will store True if the flag is provided on the command line and False otherwise).

The command line options are derived from the parameter names like so:

Options:
  -h, --help            show this help message and exit
  -l, --list-geocoders
  -a API_KEY, --api-key=API_KEY
  -g GEOCODER, --geocoder=GEOCODER

Note that the boolean --list-geocoders is a flag, not an option that sets a value.

The short option is derived from the first letter of the parameter. If that character is already in use, the second character will be used and so on.

The long option is the full name of the parameter with underscores converted to hyphens.

If you want complete control over the name of the options, simply name your parameter as follows:

def foo(q_custom_name=False):

This will result in a short option of -q and a long option of --custom-name.

Special arguments

Arguments with the names 'stdin', 'stdout' or 'stderr' will be automatically passed the relevant Python objects, for example:

#!/usr/bin/env python
# upper.py
import optfunc

@optfunc.main
def upper_stdin(stdin, stdout):
    stdout.write(stdin.read().upper())

Does the following:

$ echo "Hello, world" | ./upper.py
HELLO, WORLD

Subcommands

Some command line applications feature subcommands, with the first argument to the application indicating which subcommand should be executed.

optfunc has the beginnings of support for this - you can pass an array of functions to the optfunc.run() and the names of the functions will be used to select a subcommand based on the first argument:

import optfunc

def one(arg):
    print "One: %s" % arg

def two(arg):
    print "Two: %s" % arg

def three(arg):
    print "Three: %s" % arg

if __name__ == '__main__':
    optfunc.run([one, two, three])

Usage looks like this:

$ ./subcommands_demo.py    
Unknown command: try 'one', 'two' or 'three'
$ ./subcommands_demo.py one
one: Required 1 arguments, got 0
$ ./subcommands_demo.py two arg
Two: arg

This approach is limited in that help can be provided for an individual option but not for the application as a whole. If anyone knows how to get optparse to handle the subcommand pattern please let me know.

Decorators

optfunc also supports two decorators for stuff I couldn't work out how to shoehorn in to a regular function definition. geocode.py shows them in action:

@optfunc.notstrict
@optfunc.arghelp('list_geocoders', 'list available geocoders and exit')
def geocode(s, api_key='', geocoder='google', list_geocoders=False):
    # ...

@notstrict means "don't throw an error if one of the required positional arguments is missing" - in the above example we use this because we still want the list_geocoders argument to work even if a string has not been provided.

@arghelp('arg-name', 'help text') allows you to provide help on individual arguments, which will then be displayed when --help is called.

TODO

• Support for different argument types (int, string, filehandle, choices)
• Special handling for 'stdin' as an argument name
• Proper unix error semantics (sys.exit(1) etc)
• Allow the function to be a generator, print iterations to stdout
• Support for *args (I don't think **kwargs makes sense for optfunc)
• Subcommands need to interact with --help better