Template inheritance with extends in Flask

Introduction

Template inheritance helps you reuse common parts of web pages easily. It saves time by letting you build a base layout and add specific content on top.

You want all pages on your website to share the same header and footer.

You need to create multiple pages that look similar but have different main content.

You want to keep your HTML code clean and avoid repeating the same code in every file.

You are building a website with many pages and want to update the design quickly in one place.

Syntax

Flask

{% extends 'base.html' %}

{% block content %}
  <!-- page specific content here -->
{% endblock %}

The extends tag tells Flask which template to use as the base layout.

The block tags define sections that child templates can fill with their own content.

Examples

This example shows a child template that sets a page title and main content inside blocks.

Flask

{% extends 'layout.html' %}

{% block title %}Home Page{% endblock %}

{% block content %}
  <p>Welcome to the home page!</p>
{% endblock %}

This example overrides a sidebar block to add a custom menu.

Flask

{% extends 'base.html' %}

{% block sidebar %}
  <ul>
    <li>Link 1</li>
    <li>Link 2</li>
  </ul>
{% endblock %}

Sample Program

This example shows a base template with header, footer, and content blocks. The home.html file extends base.html and fills in the title and content blocks with page-specific content.

Flask

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 %}My Website{% endblock %}</title>
</head>
<body>
  <header>
    <h1>My Website Header</h1>
  </header>

  <main>
    {% block content %}
    <p>Default content.</p>
    {% endblock %}
  </main>

  <footer>
    <p>My Website Footer</p>
  </footer>
</body>
</html>

home.html:
{% extends 'base.html' %}

{% block title %}Home Page{% endblock %}

{% block content %}
  <p>Welcome to the home page!</p>
{% endblock %}

Important Notes

Always make sure the base template has block tags where you want child templates to insert content.

You can override as many blocks as you want in the child template.

If a block is not overridden, the base template's content for that block will show.

Summary

Template inheritance lets you create a base layout and reuse it across pages.

Use {% extends %} to inherit from a base template.

Fill in or override sections using {% block %} tags.

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