| name | jinja2-patterns |
| description | Use when the user asks about "Jinja2 templates", "Flask templates", "template inheritance", "Jinja2 macros", "Jinja2 filters", "Flask render_template", "base template", "template blocks", or needs patterns for Jinja2 template engine in Flask.
|
Jinja2 Template Patterns
Production patterns for Jinja2 templating in Flask applications.
Template Inheritance
Base Template
{# templates/base.html #}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>{% block title %}App{% endblock %}</title>
<link rel="stylesheet" href="{{ url_for('static', filename='css/base.css') }}">
{% block extra_css %}{% endblock %}
</head>
<body>
{% if current_user.is_authenticated %}
<nav>
<a href="{{ url_for('dashboard.dashboard') }}">Dashboard</a>
<a href="{{ url_for('auth.logout') }}">Logout</a>
</nav>
{% endif %}
<main>
{# Flash messages #}
{% with messages = get_flashed_messages(with_categories=true) %}
{% if messages %}
{% for category, message in messages %}
<div class="alert alert-{{ category }}">{{ message }}</div>
{% endfor %}
{% endif %}
{% endwith %}
{% block content %}{% endblock %}
</main>
<script src="{{ url_for('static', filename='js/app.js') }}"></script>
{% block extra_js %}{% endblock %}
</body>
</html>
Child Template
{# templates/clients.html #}
{% extends "base.html" %}
{% block title %}Clients - NailBook{% endblock %}
{% block extra_css %}
<link rel="stylesheet" href="{{ url_for('static', filename='css/clients.css') }}">
{% endblock %}
{% block content %}
<h1>Clients</h1>
<div class="client-list">
{% for client in clients %}
<div class="client-card">
<h3>{{ client.name }}</h3>
<p>{{ client.phone }}</p>
</div>
{% else %}
<p>No clients yet.</p>
{% endfor %}
</div>
{% endblock %}
Control Structures
Conditionals
{% if user.status == 'vip' %}
<span class="badge vip">VIP</span>
{% elif user.status == 'regular' %}
<span class="badge regular">Regular</span>
{% else %}
<span class="badge new">New</span>
{% endif %}
Loops
{% for item in items %}
<div class="{{ 'even' if loop.index is even else 'odd' }}">
{{ loop.index }}. {{ item.name }}
</div>
{% endfor %}
Loop variables: loop.index (1-based), loop.index0 (0-based), loop.first, loop.last, loop.length.
Filters
Built-in Filters
{{ name|title }} {# "john doe" → "John Doe" #}
{{ price|int }} {# 19.99 → 19 #}
{{ items|length }} {# List length #}
{{ text|truncate(50) }} {# Truncate with ellipsis #}
{{ date|default('N/A') }} {# Default if None #}
{{ html_content|safe }} {# Mark as safe HTML (no escaping) #}
{{ list|join(', ') }} {# Join list items #}
{{ date_obj|string }} {# Convert to string #}
Custom Filters
@app.template_filter('currency')
def currency_filter(value):
return f"{value:,.0f} \u20B4"
@app.template_filter('timeformat')
def timeformat_filter(time_obj):
return time_obj.strftime('%H:%M')
{{ appointment.price|currency }} {# "1,500 ₴" #}
{{ appointment.time|timeformat }} {# "14:30" #}
Forms
Basic Form
<form method="POST" action="{{ url_for('clients.add_client') }}">
<input type="hidden" name="csrf_token" value="{{ csrf_token() }}">
<label for="name">Name</label>
<input type="text" name="name" id="name" required
value="{{ request.form.get('name', '') }}">
<label for="phone">Phone</label>
<input type="tel" name="phone" id="phone" required>
<button type="submit">Add Client</button>
</form>
Select with Pre-selected Value
<select name="status">
{% for option in ['new', 'regular', 'vip'] %}
<option value="{{ option }}"
{{ 'selected' if client.status == option }}>
{{ option|title }}
</option>
{% endfor %}
</select>
Macros
Reusable template components:
{# templates/macros/forms.html #}
{% macro input(name, label, type='text', value='', required=false) %}
<div class="form-group">
<label for="{{ name }}">{{ label }}</label>
<input type="{{ type }}" name="{{ name }}" id="{{ name }}"
value="{{ value }}" {{ 'required' if required }}>
</div>
{% endmacro %}
{# Usage in templates #}
{% from "macros/forms.html" import input %}
{{ input('name', 'Client Name', required=true) }}
{{ input('email', 'Email', type='email') }}
{{ input('phone', 'Phone', type='tel', required=true) }}
Static Files
<link rel="stylesheet" href="{{ url_for('static', filename='css/base.css') }}">
<script src="{{ url_for('static', filename='js/app.js') }}"></script>
<img src="{{ url_for('static', filename='images/logo.png') }}" alt="Logo">
Always use url_for('static', ...) instead of hardcoded paths — it handles URL prefixes and cache busting correctly.
Context Variables
Variables available in all templates without explicit passing:
| Variable | Source | Description |
|---|
current_user | Flask-Login | Authenticated user object |
request | Flask | Current request object |
session | Flask | Session data |
config | Flask | App configuration |
g | Flask | Per-request globals |
| Custom | @app.context_processor | User-defined globals |
