Django Template - Increment the value of a variable

I have the following code in my template

{% set counter = 0 %}
{% for object in object_list %}
    {% if object.attr1 == list1.attr1 and object.attr2 = list2.attr2 %}
        <li><a href="{{ object.get_absolute_url }}"> Link {{counter++}} </a></li>
     {% endif %}
{% endfor %}

I setting the value of a variable using this custom tag and what I want to do is to increment the value only if the if loop is satisfied. I know {{counter++}} does not work. But how can I write a custom tag that would do the same task?

Changing the state of an object in a Django template is discouraged. You should probably bite the bullet, calculate the condition beforehand and pass extra state to the template so you can simplify the template logic.

I'm no purist in this regard by the way, but I have been bitten by the purposeful limitations of Django templates a few times. You're better off not fighting against it, in my opinion.

Being that your intention seems to be to filter out non-matching items, an alternative would be to filter out those in the view and then use {{ forloop.counter }} to sort out the link text you want. So in the view you have something like this:

new_lst = filter(lambda x: x.attr0 == attr0 and x.attr1 == attr1, lst)

And then, in your template:

{% for object in new_lst %}
   <li><a href="{{ object.get_absolute_url }}"> Link {{ forloop.counter }} </a></li>
{% endfor %}

Although this has been answered and goes against what has been said I just had a thought and can't see too much harm if perhaps you made simple counter class like

class Counter:
    count = 0

    def increment(self):
        self.count += 1
        return ''

    def decrement(self):
        self.count -= 1
        return ''

    def double(self):
        self.count *= 2
        return ''

then in your template {{ counter.increment }} {{ counter.count }} etc.