Ajax blocks in Django

'''
This module introduces viewtag concept:
funcions that act both as template tags and as views.
Example usage:
First in urls (once):
    url(r'^viewtag/([-\w]+)/$', 'viewtags.view', name='viewtag'),
and
   viewtags.autodiscover()
Then in app/viewtags.py:
    from placeforpeople.viewtags import viewtag, model_arg
    @viewtag('viewtag_test.html', [model_arg('obj', Object)]) # you can use querysets too
    def test_viewtag(request, obj, page=0):
        return {
            'obj': obj,
            'page': page,
        }
Later in templates:
    {% vt:test_viewtag campaign %}
or
    $.get('{% url viewtag "test_viewtag" %}', {page: 2, object: 5}, function(data) {
        $('body').append(data);
    });
'''
import inspect
from django import template
from django.template.loader import get_template
from django.http import Http404
from django.shortcuts import render_to_response
from django.utils.importlib import import_module
from django.conf import settings


register = template.Library()
template.builtins.append(register)

_viewtags_registry = {}


def viewtag(template, args=[]):
    def decorator(func):
        _register_tag(func, template)
        _viewtags_registry[func.__name__] = (func, template, args)

        return func
    return decorator


def view(request, viewtag_name):
    try:
        func, template_, arg_processors = _viewtags_registry[viewtag_name]
    except KeyError:
        raise Http404()

    argnames = inspect.getargspec(func).args
    kwargs = {}
    for argname in argnames:
        if argname in request.REQUEST and argname != 'request':
            kwargs[argname] = request.REQUEST[argname]
    for proc in arg_processors:
        kwargs = proc(request, kwargs)

    return render_to_response(template_, func(request, **kwargs), template.RequestContext(request))


def model_arg(key, qs_or_model):
    from django.db import models

    if isinstance(qs_or_model, models.base.ModelBase):
        qs = qs_or_model._default_manager.all()
    else:
        qs = qs_or_model

    def processor(request, kwargs):
        id = request.REQUEST.get(key) or request.REQUEST.get(key + '_id')
        if not id is None:
            kwargs[key] = qs.get(id=id)
        return kwargs
    return processor


def autodiscover():
    for app in settings.INSTALLED_APPS:
        try:
            import_module('%s.viewtags' % app)
        except ImportError:
            pass


def _register_tag(func, template):
    def tag(parser, token):
        parts = token.split_contents()
        args, kwargs = [], []
        for part in parts[1:]:
            if '=' in part:
                kwargs.append(part.split('=', 1))
            else:
                args.append(part)

        return TempNode(func, template, args, dict(kwargs))
    register.tag('vt:' + func.__name__, tag)


class TempNode(template.Node):
    def __init__(self, func, template_, args, kwargs):
        self.func = func
        self.template = get_template(template_)
        self.args = [template.Variable(arg) for arg in args]
        self.kwargs = dict([(k, template.Variable(v)) for (k,v) in kwargs.items()])

    def render(self, context):
        args = [context['request']] + [arg.resolve(context) for arg in self.args]
        kwargs = dict([
            (key, val.resolve(context)) for (key, val) in self.kwargs.items()
        ])
        context.update(self.func(*args, **kwargs))
        res = self.template.render(context)
        context.pop()
        return res