Loops

Jinja's for loops enable you to iterate over data structures such as lists, dictionaries, and tuples, to produce dynamic content. For instance, you can use a loop to generate a list of blog posts on a website or display the names of all team members on a company page.

Here is an example of Jinja loop that iterates over a list of customers named customers:

{% for customer in customers %}
	{{ customer.name }}
{% endfor %}

As with conditionals, you need to explicitly mark the end of the loop block, in this case using endfor.

Loop over dictionaries

If you are iterating over a dictionary, you need to use a slightly different syntax

{% for key, value in my_dict.items() %}
	{{ key }} = {{ value }}
{% endfor %}

Use an else statement

You can optionally also add an else statement to the loop: that statement will be used if the data structure you are iterating over is empty. For example:

{% for customer in customers %}
	{{ customer.name }}
{% else %}
	No customer.
{% endfor %}

Use loop-specific variables

Jinja provides several loop-specific variables that are available within your loop block: an example is loop.first, which is True if your loop is iterating over its first element and False in every other case. Here is a practical example:

# file: jinja-templates/03-loops.txt

{%- for customer in customers -%}
	{% if not loop.first %},{% endif %}{{ customer.name -}}
{% endfor %}

This template will print all customer names in a single line, prepending each of them with a comma with the exception of the first one.

Rendering the template with the following input

>>> tpl = env.get_template("03-loops.txt")
>>> tpl.render(customers=[{"name": "A"}, {"name": "B"}, {"name": "C"}])

produces

A,B,C

A does not have a leading comma because not loop.first returned False, since A is the first element in the list.

Avoiding unwanted commas is especially useful when using loops to build an SQL query with a dynamic list of columns. For example, the following template generates a valid SQL query, without extra trailing commas:

{% set values = [ 'Central','South','East','West'] %}
SELECT
{% for value in values %}
	'{{value}}' as {{ value }} 
    {% if not loop.last %},{% endif %}
{% endfor %}

Here is an overview of the loop-specific variables, while the complete list is available here:

Filter elements

You can use an if statement to filter the sequence during iteration, allowing you to skip those elements (if any) that do not meet the condition.

For example:

{% for customer in customers if customer.is_business %}
	{{ customer.name }}
{% else %}
	No customer.
{% endfor %}

will only return the names of those customers for which is_business == True.

Just like with regular conditional statements, you can combine multiple conditions using logical operators.

References

- https://jinja.palletsprojects.com/en/3.1.x/templates/#for