CODEWITHSUNDEEP
djangodjango-templates
Sachin
Sachin

Reputation: 3782

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?

Upvotes: 31

Views: 60347

Answers (4)

Darwin
Darwin

Reputation: 2037

For increase value in template django: if varint=6, the result will be 8

{{ varint|add:"2" }}

For decrease value in template django:

if varint=6, the result will be 5

  {{ varint|add:"-1" }}

Upvotes: 5

T I
T I

Reputation: 9933

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.

Upvotes: 16

Eduardo Ivanec
Eduardo Ivanec

Reputation: 11852

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 %}

Upvotes: 36

Sachin
Sachin

Reputation: 3782

I have just found out the answer myself. As I said I was using this custom tag, which assigns a value to a variable. What is actually does is set give a value to a variable in the context so I just retrieved the value from the context and incremented it.

Here is the code 

class IncrementVarNode(template.Node):

    def __init__(self, var_name):
        self.var_name = var_name

    def render(self,context):
        value = context[self.var_name]
        context[self.var_name] = value + 1
        return u""

def increment_var(parser, token):

    parts = token.split_contents()
    if len(parts) < 2:
        raise template.TemplateSyntaxError("'increment' tag must be of the form:  {% increment <var_name> %}")
    return IncrementVarNode(parts[1])

register.tag('increment', increment_var)

It can be used like this {% increment <var_name> %} but this var_name should have been set to a value previously using the custom tag mentioned above as {% set <var_name> = <var_value> %}

Upvotes: 1

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