Pass variables from child template to parent in Jinja2 - jinja2

Question

I want to have one parent template and many children templates with their own variables that they pass to the parent, like so:
parent.html:
{% block variables %}
{% endblock %}
{% if bool_var %}
{{ option_a }}
{% else %}
{{ option_b }}
{% endif %}
child.html:
{% extends "parent.html" %}
{% block variables %}
{% set bool_var = True %}
{% set option_a = 'Text specific to this child template' %}
{% set option_b = 'More text specific to this child template' %}
{% endblock %}
But the variables end up undefined in the parent.

Answer 1

Ah. Apparently they won't be defined when they are passed through blocks. The solution is to just remove the block tags and set it up like so:
parent.html:
{% if bool_var %}
{{ option_a }}
{% else %}
{{ option_b }}
{% endif %}
child.html:
{% extends "parent.html" %}
{% set bool_var = True %}
{% set option_a = 'Text specific to this child template' %}
{% set option_b = 'More text specific to this child template' %}

Answer 2

If Nathron's solution does not fix your problem, you can use a function in combination with a global python variable to pass a variable value.
Advantage: The variable's value will available in all templates. You can set the variable inside a block.
Disadvantage: More overhead.
This is what I did:
child.j2:
{{ set_my_var('new var value') }}
base.j2
{% set my_var = get_my_var() %}
python code
my_var = ''
def set_my_var(value):
global my_var
my_var = value
return '' # a function returning nothing will print a "none"
def get_my_var():
global my_var
return my_var
# make functions available inside jinja2
config = { 'set_my_var': set_my_var,
'get_my_var': get_my_var,
...
}
template = env.get_template('base.j2')
generated_code = template.render(config)

Answer 3

In some cases, you can avoid this 'parameter-passing' by creating another variant of the parent that adds/removes some block and extends it instead.
{% extends [condition]|yesno:'parent_1.html,parent_2.html' %}