Testable python code...

bootstrap.sh

#!/bin/bash
curl -O https://raw.github.com/pypa/virtualenv/master/virtualenv.py
python virtualenv.py testing_in_python
. testing_in_python/bin/activate
pip install django mock
python testable_templatetag.py

templatetag.py

from django.template import Node, Library, TemplateSyntaxError

register = Library()

class GetLinkNode(Node):
    def __init__(self, app, model, pk):
        self.app, self.model, self.pk = app, model, pk

    def render(self, context):
        from django.db.models.loading import get_model
        model = get_model(self.app, self.model)
        m = model._default_manager.get(pk=self.pk)
        return m.get_absolute_url()

@register.tag('get_link')
def do_get_link(parser, token):
    """ 
    {% get_link for articles.article 2 %}
    """
    bits = token.split_contents()
    if len(bits) != 4 or bits[1] != 'for' or '.' not in bits[2] or not bits[3].isdigit():
        raise TemplateSyntaxError()

    app, model = bits[2].split('.', 1)
    return GetLinkNode(app, model, int(bits[3]))

testable_templatetag.py

import unittest, mock

from django.template import Node, Library, TemplateSyntaxError

register = Library()

class GetLinkNode(Node):
    def __init__(self, app, model, pk):
        self.app, self.model, self.pk = app, model, pk

    def get_model(self):
        from django.db.models.loading import get_model
        model = get_model(self.app, self.model)
        return model._default_manager.get(pk=self.pk)

    def render(self, context):
        m = self.get_model()
        return m.get_absolute_url()

def _parse_get_link(bits):
    if len(bits) != 4 or bits[1] != 'for' or '.' not in bits[2] or not bits[3].isdigit():
        raise TemplateSyntaxError()

    app, model = bits[2].split('.', 1)
    return GetLinkNode(app, model, int(bits[3]))


@register.tag('get_link')
def do_get_link(parser, token):
    """ 
    {% get_link for articles.article 2 %}
    """
    bits = token.split_contents()
    return _parse_get_link(bits)

class TestGetLink(unittest.TestCase):
    def test_parser_raises_on_empty(self):
        self.assertRaises(TemplateSyntaxError, _parse_get_link, []) 

    def test_parser_raises_on_no_for(self):
        self.assertRaises(TemplateSyntaxError, _parse_get_link, ['get_link', 'as', 'articles.article', '1'])

    def test_parser_raises_on_no_dot_in_model(self):
        self.assertRaises(TemplateSyntaxError, _parse_get_link, ['get_link', 'for', 'article', '1'])

    def test_parser_raises_on_non_numeric_pk(self):
        self.assertRaises(TemplateSyntaxError, _parse_get_link, ['get_link', 'for', 'articles.article', 'one'])

    def test_parser_constructs_correct_node(self):
        node = _parse_get_link(['get_link', 'for', 'articles.article', '1'])

        self.assertIsInstance(node, GetLinkNode)
        self.assertEquals(1, node.pk)
        self.assertEquals('articles', node.app)
        self.assertEquals('article', node.model)

    def test_node_renders_url_of_object(self):
        node = GetLinkNode('articles','article', 1)
        with mock.patch.object(node, 'get_model'):
            ret = node.render({})
            node.get_model.assert_called_once_with()
            self.assertEquals(ret, node.get_model().get_absolute_url())

if __name__ == '__main__':
    unittest.main()