Does Jinja support variable assignment as a result of a loop? - jinja2

Question

I've been using Jinja and DBT for a month now, and despite reading a lot about it, I didn't quite figure out how to create a list from another, using a simple for loop like I would in Python.
Just a toy example:
{%- set not_wanted_columns = ['apple', 'banana'] -%}
{%- set all_columns = ['kiwi', 'peach', 'apple', 'banana', 'apricot', 'pineapple'] -%}
What I want is a list as so:
{% set filtered_columns = ['kiwi', 'peach', 'apricot', 'pineapple'] %}
Naturally, I don't want to manually write this result because the full list might be dynamic or too long. I'm not even sure if Jinja does actually support this, although I do think this is a common problem.

Answer 1

As you have probably read from the documentation:
Please note that assignments in loops will be cleared at the end of the iteration and cannot outlive the loop scope. Older versions of Jinja had a bug where in some circumstances it appeared that assignments would work. This is not supported.
Source: https://jinja.palletsprojects.com/en/3.1.x/templates/#for
And I guess when you are speaking about
using a simple for loop like I would in Python
What you mean here is using a list comprehension.
So, as showed in the documentation, Jinja is using filter to achieve this:
Example usage:
{{ numbers|select("odd") }}
{{ numbers|select("divisibleby", 3) }}
Similar to a generator comprehension such as:
(n for n in numbers if test_odd(n))
(n for n in numbers if test_divisibleby(n, 3))
Source: https://jinja.palletsprojects.com/en/3.1.x/templates/#jinja-filters.select
There are actual four of those filter acting as generator comprehension:
reject
rejectattr
select
selectattr
So, in your case, a reject filter would totally do the trick:
{%- set filtered_columns = all_columns
| reject('in', not_wanted_columns)
| list
-%}
But, if you really want, you could also achieve it in a for:
{%- for column in all_columns if column not in not_wanted_columns -%}
{% do filtered_columns.append(column) %}
{%- endfor -%}
The do statement being a way to use list.append() without oddities being printed out: https://jinja.palletsprojects.com/en/3.1.x/templates/#expression-statement.