Common Mistakes🔖

Confusing {{ }} and {% %}🔖

<!-- WRONG — this is a loop, not output -->
{{ for task in tasks }}
 
<!-- RIGHT — logic uses {% %} -->
{% for task in tasks %}

Remember: {{ }} outputs a value. {% %} runs logic. If you put a for loop inside {{ }}, you'll see the literal text "for task in tasks" on the page.

Forgetting {% endfor %} or {% endif %}🔖

{% for task in tasks %}
    <li>{{ task.title }}</li>
<!-- MISSING: {% endfor %} -->

Jinja2 will throw an error about an "unexpected end of template." Unlike Python, indentation doesn't close blocks. You must explicitly close every {% for %} with {% endfor %} and every {% if %} with {% endif %}.

Using Python syntax in Jinja2🔖

<!-- WRONG — Python dict syntax -->
{{ task["title"] }}
 
<!-- RIGHT — Jinja2 dot notation -->
{{ task.title }}

Both actually work in Jinja2, but dot notation is the convention. The real trap is trying to use Python functions that don't exist in Jinja2 — like len(tasks) instead of tasks|length.

Template variable doesn't match the route🔖

# app.py
render_template("tasks.html", task_list=tasks)

<!-- tasks.html — WRONG name -->
{% for task in tasks %}

The template uses tasks but the route passed task_list. No error — the loop just silently does nothing because tasks is undefined. Check the left side of the = in your render_template() call.

Forgetting {% extends "base.html" %}🔖

Your page renders but has no nav, no footer, no styling. It's just raw content. The template isn't inheriting from base.html.

Fix: make sure {% extends "base.html" %} is the very first line of the file. Not the second line. Not after a comment. The first line.

Content outside the block🔖

{% extends "base.html" %}
 
<h1>This heading disappears</h1>
 
{% block content %}
    <p>Only this part shows up.</p>
{% endblock %}

Anything between {% extends %} and {% block %} is silently discarded. All your content must go inside the block.

Putting templates in the wrong folder🔖

your-project/
    app.py
    index.html          ← WRONG — not in templates/
    templates/
        base.html

render_template("index.html") only looks inside templates/. If the file is anywhere else, you get TemplateNotFound.