Block Scope

Introduction

In Jinja, block scope refers to the scope in which a variable exists and is accessible. This guide will explain how block scope works in Jinja and provide examples to illustrate its behavior.

What is Block Scope?

Block scope in Jinja is best understood by looking at an example. Consider the following code snippet:

{% set var = 0 %}
{%- for number in range(1, 10) -%}
    {% set var = var + 1 %}
{% endfor %}
{{ var }}

In many programming languages, you might expect the result to be 9, but in Jinja, the output will be 0. This is because, in Jinja, anything defined within a for loop stays within that loop. When you use the {% set %} feature within a loop, you're actually creating a new variable with the same name, and this variable is destroyed when the loop ends.

Exception: Appending to a List

There is an exception to this block scope rule, which is when you are appending to a list that already exists. This works because you are modifying the actual variable that was created before the loop is called, rather than creating a new variable with the same name within the loop. Here's an example:

{% set var = [] %}
{%- for number in range(1, 10) -%}
    {% do var.append(number) %}
{% endfor %}
{{ var }}

In this case, the code will result in a list containing the numbers 1 to 9.

Persistent Scope

To maintain a persistent scope in Jinja and have variables that persist across multiple blocks or loops, you can create a namespace. Think of this namespace as a data structure that holds your variables. Here's an example:

{% set var = namespace(value=0) %}
{%- for number in range(1, 10) -%}
    {% set var.value = var.value + 1 %}
{% endfor %}
{{ var.value }}

In this code, we create a namespace named var with an initial value of 0. Inside the loop, we can access and modify var.value, and the changes persist outside the loop. This code will result in the expected value of 9.

It's important to note that in the example above, value in namespace(value=0)is another variable that exists within the var namespace, meaning it's an arbitrary name. Something like this would function in the same exact way:

{% set var = namespace(data=0) %}
{%- for number in range(1, 10) -%}
    {% set var.data = var.data + 1 %}
{% endfor %}
{{ var.data }}

You're also not restricted to a single variable within the namespace. You can mix datatypes within the namespace, just like you can in other data structures. Something like this would work as well:

{% set var = namespace(data=0, value=1) %}
{%- for number in range(1, 10) -%}
    {% set var.data = var.data + 1 %}
    {% set var.value = var.value + 1 %}
{% endfor %}
{{ var.data }}
{{ var.value }}

Conclusion

Understanding block scope in Jinja is crucial for effectively working with variables within templates. Remember that variables defined within a block, such as a for loop, are limited to that block's scope. To maintain persistent scope or modify variables across blocks, consider using namespaces as demonstrated in this documentation.