Template inheritance with extends in Flask - Mini Project: Build & Apply

Template inheritance with extends in Flask

📖 Scenario: You are building a simple website using Flask. You want to create a consistent look for all pages by using a base HTML template. Then, you will create a child template that inherits from the base template and fills in the unique content for the homepage.

🎯 Goal: Build two HTML templates: a base template called base.html with a header and footer, and a child template called home.html that extends base.html and adds a welcome message in the content area.

📋 What You'll Learn

Create a base template base.html with a block content placeholder

Create a child template home.html that uses {% extends 'base.html' %}

Override the content block in home.html with a welcome message

Use semantic HTML tags for header, main content, and footer

💡 Why This Matters

🌍 Real World

Template inheritance helps keep your website consistent and easy to maintain by reusing common layout code.

💼 Career

Understanding template inheritance is essential for web developers working with Flask or other template-based frameworks to build scalable web apps.

1

Create the base template base.html

Create a file called base.html with a basic HTML5 structure. Inside the <body>, add a <header> with the text "My Website Header" and a <footer> with the text "My Website Footer". Between them, add a Jinja2 block named content using {% block content %}{% endblock %}.

Flask

    
    
    Base Template


    My Website Header
    {% block content %}{% endblock %}
    My Website Footer

Hint

Use {% block content %}{% endblock %} inside the body between header and footer.

2

Create the child template home.html that extends base.html

Create a file called home.html. At the top, write {% extends 'base.html' %} to inherit from the base template. Then, open a content block using {% block content %}.

Flask

{% extends 'base.html' %}

{% block content %}

Hint

Start your file with {% extends 'base.html' %} and then open the content block.

3

Add a welcome message inside the content block

Inside the content block in home.html, add a <main> tag with a <h1> that says "Welcome to the Homepage!". Then close the content block with {% endblock %}.

Flask

{% extends 'base.html' %}

{% block content %}

Hint

Wrap the welcome message in a <main> tag and close the block with {% endblock %}.

4

Use the templates in a Flask route

In your Flask app file, import Flask and render_template. Create an app instance. Define a route for / that returns render_template('home.html'). Make sure to include the if __name__ == '__main__': block to run the app.

Flask

from flask import Flask, render_template

app = Flask(__name__)

# Define route for homepage

# Your code here

if __name__ == '__main__':
    app.run(debug=True)

Hint

Use @app.route('/') and a function that returns render_template('home.html').

Practice

1. What is the main purpose of using {% extends %} in Flask templates?

easy

A. To inherit the layout from a base template and reuse it

B. To include JavaScript files in the template

C. To define a new CSS style inside the template

D. To create a new Flask route

Solution

1. Step 1: Understand the role of {% extends %}

   This tag is used to inherit from a base template, allowing reuse of common layout parts.

2. Step 2: Differentiate from other template tags

   Other options like including scripts or defining CSS are done differently, not with {% extends %}.

3. Final Answer:

   To inherit the layout from a base template and reuse it -> Option A

4. Quick Check:

   Template inheritance = reuse layout [OK]

Hint: Remember: extends means reuse base layout [OK]

Common Mistakes:

• Confusing extends with including scripts
• Thinking extends creates new routes
• Using extends to add CSS styles

2. Which of the following is the correct syntax to inherit from a base template named base.html?

easy

A. {% include 'base.html' %}

B. {% import 'base.html' %}

C. {% extends 'base.html' %}

D. {% block 'base.html' %}

Solution

1. Step 1: Recall the syntax for template inheritance

   The correct tag to inherit a template is {% extends 'filename' %}.

2. Step 2: Identify incorrect tags

   {% include %} inserts content but does not inherit; {% import %} is for macros; {% block %} defines sections, not inheritance.

3. Final Answer:

   {% extends 'base.html' %} -> Option C

4. Quick Check:

   Extends = inherit base template [OK]

Hint: Use extends, not include, to inherit templates [OK]

Common Mistakes:

• Using include instead of extends
• Confusing import with extends
• Misusing block tag for inheritance

3. Given the base template base.html with a block named content and a child template that extends it and overrides content, what will be displayed when rendering the child template?

medium

A. An error because blocks cannot be overridden

B. The content defined inside the child's content block

C. The content defined inside the base template's content block

D. Both base and child content concatenated automatically

Solution

1. Step 1: Understand block overriding in template inheritance

   The child template replaces the base template's block content with its own block content.

2. Step 2: Clarify behavior of blocks

   Blocks are placeholders in the base template; child templates override them. No automatic concatenation occurs unless explicitly coded.

3. Final Answer:

   The content defined inside the child's content block -> Option B

4. Quick Check:

   Child block overrides base block content [OK]

Hint: Child block content replaces base block content [OK]

Common Mistakes:

• Thinking base block content shows instead
• Expecting automatic concatenation of blocks
• Believing block override causes errors

4. Consider this child template code snippet:

{% extends 'base.html' %}
{% block content %}
Hello World

What is the error in this template?

medium

A. The closing tag should be {% endblock %} instead of

B. The extends tag is missing quotes around the filename

C. The block name should be main not content

D. The p tag cannot be used inside blocks

Solution

1. Step 1: Check block syntax correctness

   Blocks must be closed with {% endblock %}, not with HTML-like closing tags.

2. Step 2: Verify other parts

   The extends tag is correctly quoted; block name can be any valid name; p tags are allowed inside blocks.

3. Final Answer:

   The closing tag should be {% endblock %} instead of -> Option A

4. Quick Check:

   Block ends with endblock tag [OK]

Hint: Close blocks with {% endblock %}, not HTML tags [OK]

Common Mistakes:

• Using HTML closing tags for blocks
• Forgetting quotes in extends (not here though)
• Assuming block names are fixed keywords

5. You have a base template base.html with two blocks: header and content. You want a child template to reuse the base layout but only change the content block, keeping the base header as is. Which is the correct approach?

hard

A. In the child template, extend base.html and redefine only the header block

B. In the child template, extend base.html and redefine both header and content blocks

C. In the child template, do not use extends; copy all base content and change content

D. In the child template, extend base.html and define only the content block; omit the header block

Solution

1. Step 1: Understand block overriding behavior

   If a block is not redefined in the child, the base template's block content is used automatically.

2. Step 2: Apply this to the problem

   To keep the base header, do not redefine the header block in the child; only override content.

3. Final Answer:

   In the child template, extend base.html and define only the content block; omit the header block -> Option D

4. Quick Check:

   Omit block to keep base content [OK]

Hint: Override only blocks you want to change [OK]

Common Mistakes:

• Redefining all blocks unnecessarily
• Copying base template instead of extending
• Overriding wrong block by mistake