Template fragment caching in Django - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is template fragment caching in Django?

Template fragment caching stores parts of a template's rendered output to speed up page loading by reusing cached HTML instead of rendering it again.

beginner

How do you mark a part of a Django template for fragment caching?

Use the {% cache %} template tag with a cache timeout and a cache key to wrap the part you want to cache.

beginner

Example of using {% cache %} tag in Django template?

Example: {% cache 600 sidebar %} ... HTML for sidebar ... {% endcache %} caches the sidebar for 600 seconds.

intermediate

Why use template fragment caching instead of full page caching?

Fragment caching lets you cache only slow or complex parts of a page, allowing dynamic parts to update without caching the whole page.

intermediate

What happens if the cache key changes in template fragment caching?

If the cache key changes, Django treats it as a new cache entry and renders the fragment again, storing the new output.

Which Django template tag is used for fragment caching?

A {% loadcache %}

B {% cache %}

C {% fragment %}

D {% storecache %}

The {% cache %} tag is used to mark template fragments for caching.

What does the number in {% cache 300 key %} represent?

A Cache priority level

B Cache key length

C Number of times to cache

D Cache timeout in seconds

The number 300 means the fragment is cached for 300 seconds (5 minutes).

If a cached fragment is expired, what does Django do?

A Skips rendering the fragment

B Shows an error

C Renders the fragment again and caches it

D Uses the old cached version anyway

Django re-renders the fragment and stores the new output in cache.

Can template fragment caching improve performance when only part of a page is slow?

A Yes, by caching just the slow parts

B No, it caches the whole page

C No, it slows down rendering

D Yes, but only for static pages

Fragment caching targets slow parts to speed up page load without caching everything.

What must you do to use {% cache %} tag in your Django template?

A Load the cache template tag library with {% load cache %}

B Import cache in views.py

C Enable caching in settings.py only

D No setup needed

You must load the cache tag library in the template using {% load cache %} before using {% cache %}.

Explain how to implement template fragment caching in a Django template and why it helps performance.

Think about wrapping slow parts of your template with a special tag.

Describe what happens when a cached fragment expires and how Django handles it.

Consider what Django does when the cache is no longer valid.

Practice

1. What is the main purpose of using {% cache %} in Django templates?

easy

A. To permanently store user data in the database

B. To save a part of the page output and reuse it to speed up loading

C. To encrypt sensitive information in the template

D. To validate form inputs before rendering

Solution

1. Step 1: Understand what template fragment caching does

   Template fragment caching stores the rendered output of a part of a template to avoid re-rendering it every time.

2. Step 2: Identify the purpose of the {% cache %} tag

   The {% cache %} tag is used to wrap parts of a template that should be cached for faster page loads.

3. Final Answer:

   To save a part of the page output and reuse it to speed up loading -> Option B

4. Quick Check:

   Template fragment caching = save and reuse output [OK]

Hint: Cache stores parts of page output to speed loading [OK]

Common Mistakes:

• Thinking cache stores user data permanently
• Confusing cache with encryption
• Using cache tag for form validation

2. Which of the following is the correct syntax to cache a template fragment for 300 seconds with key 'sidebar'?

easy

A. {% cache 300 sidebar %} ... {% endcache %}

B. {% cache sidebar 300 %} ... {% endcache %}

C. {% cache 'sidebar' 300 %} ... {% endcache %}

D. {% cache 300 'sidebar' %} ... {% endcache %}

Solution

1. Step 1: Recall the correct order of arguments in the cache tag

   The syntax is {% cache timeout key %} where timeout is an integer and key is a string.

2. Step 2: Match the syntax with the options

   {% cache 300 'sidebar' %} ... {% endcache %} uses 300 as timeout and 'sidebar' as key in correct order and quotes.

3. Final Answer:

   {% cache 300 'sidebar' %} ... {% endcache %} -> Option D

4. Quick Check:

   Cache syntax = timeout then quoted key [OK]

Hint: Timeout first, then quoted key in cache tag [OK]

Common Mistakes:

• Putting key before timeout
• Not quoting the cache key string
• Using variable name without quotes

3. Given this template code:

{% cache 600 'menu' user.id %}

  
Home
  
Profile

{% endcache %}

What happens if user.id changes?

medium

A. A new cache entry is created for the new user.id

B. The cached fragment is reused regardless of user.id

C. The cache is cleared completely

D. An error occurs because user.id cannot be used

Solution

1. Step 1: Understand cache key with extra parameters

   Extra parameters after the key string are used to create a unique cache key per value.

2. Step 2: Effect of changing user.id on cache

   When user.id changes, Django creates a new cache entry for that user, so the fragment is cached separately.

3. Final Answer:

   A new cache entry is created for the new user.id -> Option A

4. Quick Check:

   Cache key + params = unique cache per user [OK]

Hint: Extra params create unique cache keys [OK]

Common Mistakes:

• Assuming cache ignores extra parameters
• Thinking cache clears all entries on param change
• Believing user.id cannot be used in cache tag

4. Identify the error in this template fragment caching usage:

{% cache 'sidebar' 300 %}
Sidebar content
{% endcache %}

medium

A. The timeout and key order is reversed; timeout must come first

B. Missing closing tag {% endcache %}

C. The cache key must be an integer, not a string

D. Cache tag cannot wrap HTML elements

Solution

1. Step 1: Check the order of arguments in the cache tag

   The correct order is timeout (integer) first, then cache key (string).

2. Step 2: Identify the mistake in the given code

   The code uses 'sidebar' first and 300 second, which is reversed.

3. Final Answer:

   The timeout and key order is reversed; timeout must come first -> Option A

4. Quick Check:

   Timeout first, key second in cache tag [OK]

Hint: Timeout always before key in cache tag [OK]

Common Mistakes:

• Swapping timeout and key order
• Forgetting to close cache tag
• Thinking cache tag can't wrap HTML

5. You want to cache a sidebar that shows user-specific data but also updates every 10 minutes. Which is the best way to use template fragment caching?

hard

A. Do not use cache because user data changes

B. {% cache 600 'sidebar' %} ... {% endcache %} to cache once for all users

C. {% cache 600 'sidebar' user.id %} ... {% endcache %} to cache per user for 10 minutes

D. {% cache 'sidebar' 600 user.id %} ... {% endcache %} with wrong argument order

Solution

1. Step 1: Understand caching user-specific data

   To cache user-specific content, include a unique user identifier in the cache key.

2. Step 2: Set cache timeout to 600 seconds (10 minutes)

   Use 600 seconds as timeout to update cache every 10 minutes.

3. Step 3: Verify correct syntax and usage

   {% cache 600 'sidebar' user.id %} ... {% endcache %} to cache per user for 10 minutes uses correct syntax with timeout first, key string second, and user.id as extra parameter.

4. Final Answer:

   {% cache 600 'sidebar' user.id %} ... {% endcache %} to cache per user for 10 minutes -> Option C

5. Quick Check:

   User-specific cache with timeout = {% cache 600 'sidebar' user.id %} ... {% endcache %} to cache per user for 10 minutes [OK]

Hint: Use user.id param and timeout for user-specific cache [OK]

Common Mistakes:

• Caching once for all users ignoring user.id
• Swapping timeout and key order
• Avoiding cache for user data unnecessarily