colltype_examples.py

## Here are some actually-real situations where I've wanted this:
## set (dedup args)
parser.add_argument('foo', nargs="+", type=int, colltype=set)
parser.parse_args(['1','1','2','2','3','3','3'])
# Namespace(foo={1,2,3})

## sorted (sort args)
parser.add_argument('foo', nargs="+", type=int, colltype=sorted)
parser.parse_args(['10', '5', '30', '14'])
# Namespace(foo=[5, 10, 14, 30])

## tuple (avoid mutable object!)
parser.add_argument('foo', type=int, colltype=tuple)
parser.parse_args(['1','2'])
# Namespace(foo=(1,2,))

## Ranges (I'd like to at-parse-time check that a <= b)
def fwds_range(args):
    if args[0] > args[1]:
        raise ArgumentTypeError("beginning of range must be smaller than end")

    return range(*args)

parser.add_argument('foo', nargs=2, type=int, colltype=fwds_range)
parser.parse_args(['5','10'])

# This has broader applications for any case where args have some kind of 
# relation to each other that should be enforced

## Dates: Give a date or time as parts and then collect them all into a date)
from datetime import date
parser.add_argument('foo', nargs=3, type=int, colltype=lambda d: date(*d))
parser.parse_args(['2011', '12', '2'])
# Namespace(foo=datetime.Date(2011,12,2))

## Key value pairs where the value has a different type
def kv(args):
    if len(args) % 2 != 0:
        raise ArgumentTypeError("number of arguments for foo must be even")
    return {str(k):int(v) for k,v in zip(*[iter(args)] * 2)}

parser.add_argument('foo', nargs='+', colltype=kw)
parser.parse_args(["bar", "3", "baz", "2"])
# Namespace(foo={'bar': 3, 'baz': 2})