Fun with crossbreeding.

demo.py

from guard import Guard
import webob
from webob import Request
import webob.exc



def method(*method_list):
    def inner(request, *args, **kwargs):
        return request.method in method_list
    return inner


def method_authorized(*method_list):
    def inner(request, *args, **kwargs):
        return method(*method_list)(request, *args, **kwargs)\
               and request.remote_user is not None
    return inner

def method_unauthorized(*method_list):
    def inner(request, *args, **kwargs):
        return method(*method_list)(request, *args, **kwargs)\
               and request.remote_user is None
    return inner


view = Guard()

@view.when(method("GET"))
def _(request):
    print "GET"


@view.when(method_authorized("PUT"))
def _(request):
    print "PUT"


@view.when(method_authorized("DELETE"))
def _(request):
    print "DELETE"


@view.when(method_unauthorized("PUT", "DELETE"))
def _(request):
    raise webob.exc.HTTPUnauthorized()


@view.otherwise
def _(request):
    raise webob.exc.HTTPNotImplemented()


view(webob.Request.blank("/", method="GET"))
view(Request.blank("/", method="PUT", remote_user="eric"))
view(Request.blank("/", method="DELETE", remote_user="eric"))

try:
    view(webob.Request.blank("/", method="PUT"))
    assert False
except webob.exc.HTTPUnauthorized, error:
    print error.status

try:
    view(webob.Request.blank("/", method="DELETE"))
    assert False
except webob.exc.HTTPUnauthorized, error:
    print error.status

try:
    view(webob.Request.blank("/", method="POST"))
    assert False
except webob.exc.HTTPNotImplemented, error:
    print error.status

factoral.py

from guard import Guard

factoral = Guard()

@factoral.when(lambda n, accum=1: n <= 1)
def factoral(n, accum=1):
    return accum


@factoral.otherwise
def factoral(n, accum=1):
    return factoral(n - 1, accum * n)

guard.py

import webob
from webob import Request
import webob.exc


class NoMatch(Exception):
    pass

class Guard(object):
    def __init__(self):
        self.case_list = []
        self._otherwise = None

    def when(self, test):
        def dec(function):
            self.case_list.append((test, function))
            return self
        return dec

    def otherwise(self, function):
        self._otherwise = function
        return self
        
    def __call__(self, *args, **kwargs):
        for test, function in self.case_list:
            if(test(*args, **kwargs)):
                return function(*args, **kwargs)

        if self._otherwise:
            return self._otherwise(*args, **kwargs)
        
        raise NoMatch()