In Jinja, a for loop is used to iterate over a sequence of elements, such as a list or a dictionary. The syntax for a for loop in Jinja is similar to the syntax for a for loop in Python.
There are no while or other types loops in Jinja. Even though a recursion could technically be built using macros.
Jinja loops are especially useful for generating dynamic SQL queries that change based on user input or other variables. For example, a data analyst might use a Jinja loop to generate a SQL query that filters data based on user-specified criteria, such as a specific date range or a specific set of columns to include in the results.
Here is an example of how a for loop would work in Jinja:
{% set values = [ 'Central','South','East','West'] %}
SELECT
{% for value in values %}
'{{value}}' as {{ value }},
{% endfor %}
Note: The above query returns an error. Lear about Loop Variables below.
In this example, value is a variable that takes on each value in the values one at a time, starting with the first element and ending with the last element. The block of code inside the for loop (in this case, {{ value }}) is executed for each iteration of the loop.
The resulting SQL Query generates four columns. Each with the same value as the name of the column.
Once inside a loop, you have the loop object avaible to use information that is specific to your current loop.
These can be called in the usual Jinja fashion, e.g.:
- {{ loop.index }},
- {% if loop.first %}...
Variable | Description |
|---|---|
loop.index | The current iteration of the loop. (1 indexed) |
loop.revindex | The number of iterations from the end of the loop (1 indexed) |
loop.first | True if first iteration. |
loop.last | True if last iteration. |
loop.length | The number of items in the sequence. |
loop.cycle | Cycle through a given list of strings or variables. E.g.: {{ loop.cycle('odd', 'even') }} |
loop.previtem | The item from the previous iteration of the loop. Undefined during the first iteration. |
loop.nextitem | The item from the following iteration of the loop. Undefined during the last iteration. |
loop.changed(*val) | True if previously called with a different value (or not called at all). |
To fix the SQL Query from the previous example, we need to use either:
- loop.first or
- loop.last
{% set values = [ 'Central','South','East','West'] %}
SELECT
{% for value in values %}
'{{value}}' as {{ value }}
{% if not loop.last %},{% endif %}
{% endfor %}
Looping over a Dictionary:
{% for key in dict %}
{{ key }}: {{ dict[key] }}
{% endfor %}
Compare loop value to previous or next iteration:
{% for value in values %}
{% if loop.previtem is defined and value > loop.previtem %}
The value just increased!
{% endif %}
{{ value }}
{% if loop.nextitem is defined and loop.nextitem > value %}
The value will increase even more!
{% endif %}
{% endfor %}
Loop only if a Condition is met:
{% for value in values if values|length > 5%}
{{ value }}
{% endfor %}
- https://jinja.palletsprojects.com/en/3.1.x/templates/#for
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