Control Structures, Loops & Conditionals0%
Template Inheritance & Layout Composition

Control Structures, Loops & Conditionals

Beginner12 min read•Updated: Jul 11, 2026
Study MaterialsCheatsheet

Control Structures, Loops & Conditionals in Jinja2

Dynamic web pages require conditional branching and iterative rendering. Jinja2 provides robust control structures enclosed within {% ... %} blocks, including if/elif/else conditions and feature-rich for loops.

1. Conditionals (if, elif, else)

Jinja2 conditionals mirror Python's logical evaluation rules, checking for truthy and falsy values:

HTML
{% if user.is_authenticated %}
<div class="user-menu">
<span>Signed in as <strong>{{ user.username }}</strong></span>
{% if user.role == 'admin' %}
<a href="/admin" class="btn btn-warning">Admin Panel</a>
{% elif user.role == 'instructor' %}
<a href="/portal" class="btn btn-info">Teacher Portal</a>
{% else %}
<a href="/student" class="btn btn-primary">My Courses</a>
{% endif %}
<a href="/logout">Logout</a>
</div>
{% else %}
<div class="auth-buttons">
<a href="/login">Log In</a>
<a href="/register">Create Account</a>
</div>
{% endif %}

You can combine conditions with boolean operators and, or, not, and identity checks is defined, is none:

HTML
{% if discount > 0 and not is_expired %}
<span class="badge">Special Offer Active!</span>
{% endif %}

2. Iteration with for Loops

Iterate over lists, tuples, dictionaries, and database querysets:

HTML
<ul class="courses-list">
{% for course in courses %}
<li class="course-card">
<h3>{{ course.title }}</h3>
<p>Instructor: {{ course.instructor }}</p>
<span class="price">₹{{ course.price }}</span>
</li>
{% endfor %}
</ul>

Empty List Handling with {% else %}

Jinja2 provides a built-in else block inside for loops that renders when the collection is empty:

HTML
<ul class="notifications">
{% for alert in user.notifications %}
<li>{{ alert.message }} ({{ alert.date }})</li>
{% else %}
<li class="empty-state">No new notifications. You are all caught up!</li>
{% endfor %}
</ul>

3. The Special loop Variable

Inside every {% for %} loop, Jinja2 provides an automatic loop context variable containing execution metadata:

VariableTypeDescription
loop.indexint1-based iteration counter (1, 2, 3...)
loop.index0int0-based iteration counter (0, 1, 2...)
loop.revindexintIterations remaining until the end (1-based)
loop.firstboolTrue during the first iteration
loop.lastboolTrue during the final iteration
loop.lengthintTotal number of items in the iterable sequence
loop.cyclemethodAlternate between arguments: loop.cycle('odd', 'even')

Practical Example: Zebra-Striped Tables & Badges

HTML
<table class="table">
<thead>
<tr>
<th>#</th>
<th>Student Name</th>
<th>Enrolled Date</th>
<th>Action</th>
</tr>
</thead>
<tbody>
{% for student in students %}
<tr class="{{ loop.cycle('bg-white', 'bg-slate-50') }}">
<td>{{ loop.index }}</td>
<td>
{{ student.name }}
{% if loop.first %}
<span class="badge bg-gold">Top Student</span>
{% endif %}
</td>
<td>{{ student.enrolled_at }}</td>
<td><a href="/students/{{ student.id }}">View</a></td>
</tr>
{% endfor %}
</tbody>
</table>
Interactive Quiz

Practice Quiz

Question 1
What statement handles an empty list inside a Jinja2 {% for %} loop without requiring a separate if check?
Question 2
What is the value of loop.index during the very first iteration of a Jinja2 loop?
Question 3
How do you alternate row CSS classes between 'odd' and 'even' in a Jinja2 loop?
Question 4
Which boolean expression checks if a variable user_bio is explicitly defined in the template context?
Question 5
How must an {% if %} conditional block be terminated in Jinja2?
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