Template tag filters to call an instance method with 'user' parameter

users_extras.py

from django import template

register = template.Library()

@register.filter
def with_user(instance, user):
    """
    stores the user in the instance attribute '_onuser' wether exists or not.
    """
    setattr(instance, "_onuser", user)
    return instance

@register.filter
def call_method(instance, method):
    """
    calls the instance method 'method' passing the argument 'user' with the instance
    variable '_onuser'.
    Usage: {{ instance|with_user:user|call_method:"my_method_name" }}
    """
    method = getattr(instance, method)
    if hasattr(instance, "_onuser"):
        try:
            to_return = method(user=instance._onuser)
        except:
            to_return = method()
        return to_return
    return method()

users_extras_test.py

from django.test import TestCase
from django.template import Template, Context
from django.contrib.auth.models import User

class OnUserToTest():
    def usernames(self, user=None):
        basic = ['basico']
        if user:
            basic.append(user.username)
        return basic

class OnUserTestCase(TestCase):
    def test_with_user_method(self):
        u = User(username='prueba')
        c = Context({ 'test': OnUserToTest(), 'user':u})
        t = Template ("{% load users_extras %}{% for username in test|with_user:user|call_method:'usernames' %}{{username}}{% endfor %}")
        r = t.render(c)
        self.assertEquals(r, 'basicoprueba' )

    def test_no_parameters(self):
        t = Template ("{% load users_extras %}{% for username in test.usernames %}{{username}}{% endfor %}")
        u = User(username='prueba')
        c = Context({ 'test': OnUserToTest(), 'user':u})
        r = t.render(c)
        self.assertEquals(r, 'basico' )