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Flask's Jinja nesting some rendered elements in 'strong' elements

I'm experiencing a weird behavior with Jinja. I made a dynamic flask route and so I made a jinja modular template, it's just a for loop to create an element for each article present in some data (in a dict) I give to Jinja, the template looks like this :
{% for theme in article_data %}
{% for article in theme["article"] %}
{% if article["main"] == 1 %}
<div style="background-image: url('{{article['content']['image1']}}');" class="theme-item-bg frow space-between">
{% endif %}
{% endfor %}
<div class="wrapper-row space-between pinkfilter">
<div class="uB theme-item-text">{{theme["name"]}}</div>
<div class="pageChanger waves-effect waves-light btn uL primaryB" page="/nos-articles/{{theme['name']}}" title="{{theme['name']}}">Voir plus d'articles</div>
</div>
</div>
{% endfor %}
It does work correctly for most of my pages but for one, it have a really weird behavior, Jinja render one of the article correctly and nest the others in a strong element.
The data used to render the page have the same structure and is correctly parsed.
Is there a way to prevent Jinja from nesting stuff in a strongelement?

There must be either some html inside theme["name"] (fix it by escaping it with theme["name"]|escape), or a <strong> tag not closed in one your templates.
Jinja doesn’t insert random html tags, but the browsers do when trying to parse and fix a broken html code

Tornado webapp page has elements from extended template but missing all other elements

I'm creating a webapp with Tornado, and I'm having this issue where an .html page I have is extending another .html page like so:
{%extends "sitebase.html" %}
<div id="about">
<p>hello</p>
</div>
When I serve this page, I see the correct content from 'sitebase.html' (a simple nav bar), however I don't see the 'hello' on the web page. I've inspected the page and don't even see an element with this 'hello', and there are no errors in the console. When I remove the extends block and just paste the contents of sitebase.html I see the expected result.
Has anyone else had this experience with tornado where using an extend block causes other pieces of the page to be not rendered?

From docs (emphasis mine):
{% extends *filename* %}
Inherit from another template. Templates that use extends should contain one or more block tags to replace content from the parent template. Anything in the child template not contained in a block tag will be ignored. For an example, see the {% block %} tag.
This means that when you extend a template, the content within a {% block %} tag is rendered, anything outside the tag is ignored.
Example:
base.html:
<title>{% block title %}Default title{% end %}</title>
{% block content %}
{% end %}
mytemplate.html:
{% block title %}Title for this page{% end %}
{% block content %}
Some data to render in the content block
{% end %}
This will not be rendered because it is outside a block tag

what does {% random code here %} means in HTML file

I was given an assignment and I came across this line of code in one of HTML file of problem set. What does {% random code here %} means in HTML, is it a comment or what? I tried google but could not find it.
one TODO looks like this:
{% extends "layout.html" %}
{% block body %}
<div class="col">
<form action="/compare" enctype="multipart/form-data" method="post">
<!-- TODO -->
</form>
</div>
{% endblock %}
So please sort it out what it is for me ?

It is probably some kind of templating, for example:
http://jinja.pocoo.org/
This means that the {% random code here %} is meant to be filled out by a webserver before returning proper HTML.

{} is an object. Most likely this is part of a template that will process some code from a backend language or javascript. Take a look at mustache which utilizes this syntax https://mustache.github.io/.

How do you declare python variables within flask templates?

I've got some basic experience building websites using a LAMP stack. I've also got some experience with data processing using Python. I'm trying to get a grip on the mongodb-flask-python thing, so I fired everything up using this boilerplate: https://github.com/hansonkd/FlaskBootstrapSecurity
All is well.
To experiment, I tried declaring a variable and printing it...
I get this error message:
TemplateSyntaxError: Encountered unknown tag 'x'. Jinja was looking for the following tags: 'endblock'. The innermost block that needs to be closed is 'block'.
Here's my main index.html page
{% extends "base.html" %}
{% block content %}
<div class="row">
<div class="col-xs-12">
Hello World, at {{ now }}, {{ now|time_ago }}
</div>
</div>
<div class="row-center">
<div class="col">
{% x = [0,1,2,3,4,5] %}
{% for number in x}
<li> {% print(number) %}
{% endfor %}
</div>
</div>
{% endblock %}
I love learning new things, but man, can I ever get hung up for hours on the simplest of things... any help would be greatly appreciated!!!

Flask uses Jinja as its default templating engine.
The templating language is python-esque, but is not python. This is different from something like a phtml file, which is php interspersed with html.
Check the jinja documentation for more of what you can do, but here's how you set a variable within a template:
{% set x = [0,1,2,3,4,5] %}
http://jinja.pocoo.org/docs/2.9/templates/#assignments

Try this:
{% set x = [0,1,2,3,4,5] %}
See Jinja docs.

Is it possible to load a custom template tag in base and use it in extented templates

I loaded a custom template tag note_extras.py in base.html.
base.html
<div id="wrap">
{% load note_extras %}
{% block content %}
{% endblock %}
</div><!--wrap-->
but it is not accessible at templates which is an extend of base.html ie::
home.html
{% extends "base.html" %}
{% block content %}
<div class="container">
{% create_tagmenu request.user.pk %}
</div>
{% endblock %}
it is working fine if i load note_extras in home.html ie:
{% extends "base.html" %}
{% load note_extras %}
....

In Django template language, you must load all needed template libraries in each of the templates.
I personally think it is a good idea because it makes templates more explicit (which is better than implicit). Ill give an example. Prior to Django 1.5, the default behavior for a url tag was to provide the view name in plaintext as well as all the needed parameters:
{% url path.to.view ... %}
There however was no way to provide the path to the view via a context variable:
{% with var='path.to.view' %}
{% url var ... %}
{% endwith %}
To solve that, starting with 1.3, you could import the future version of the url tag (which became the default in 1.5) by doing:
{% load url from future %}
{% url var ... %}
or
{% url 'path.to.view' ... %}
Now imagine that you would need to create a template which would extend from a base template which you did not create (e.g. one of django admin base templates). Then imagine that within the base template it would have {% load url from future %}. As a result, {% url path.to.view ... %} within your template would become invalid without any explicit explanation.
Of course this example does not matter anymore (starting with 1.5) however hopefully it illustrates a point that being explicit in templates is better than implicit which is why the currently implementation is the way it is.

If you want that a template tag is loaded in every template you want to do it in the init file of your app:
from django.template.loader import add_to_builtins
add_to_builtins('my_app.templatetags.note_extras')

In case anyone was wondering, add_to_builtins has been deprecated but one could still load a tag for all of the templates in the project via settings.TEMPLATES - supported for Django 1.9 onwards as described here:
https://stackoverflow.com/a/59719364/2447803