Low-level cache API in Django - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of Django's low-level cache API?

It allows you to store and retrieve data directly in the cache, giving you fine control over caching behavior beyond automatic caching.

beginner

Which function is used to add a new key-value pair to the cache only if the key does not already exist?

cache.add(key, value, timeout=None) adds the key only if it is not already in the cache.

beginner

How do you retrieve a value from the cache with a default if the key is missing?

Use cache.get(key, default=None). It returns the cached value or the default if the key is not found.

beginner

What does cache.set(key, value, timeout=None) do?

It stores the value under the given key in the cache. The optional timeout sets how long the value stays cached.

beginner

How can you delete a cached value using the low-level cache API?

Use cache.delete(key) to remove the value associated with the key from the cache.

Which Django cache function adds a key only if it does not exist?

A cache.add()

B cache.set()

C cache.get()

D cache.delete()

cache.add() adds a key only if it is not already present in the cache.

What happens if you use cache.get() with a key that is not in the cache and no default is provided?

A Returns an empty string

B Raises an error

C Returns None

D Returns False

cache.get() returns None if the key is missing and no default is given.

How do you specify how long a value stays in the cache when using cache.set()?

A By setting the <code>duration</code> parameter

B By setting the <code>limit</code> parameter

C By setting the <code>expiry</code> parameter

D By setting the <code>timeout</code> parameter

The timeout parameter controls how many seconds the value stays cached.

Which function removes a key and its value from the cache?

A cache.delete()

B cache.pop()

C cache.clear()

D cache.remove()

cache.delete() removes the key and its value from the cache.

If you want to update a cached value regardless of whether it exists, which function do you use?

A cache.get()

B cache.set()

C cache.add()

D cache.update()

cache.set() stores or updates the value for a key in the cache.

Explain how to store, retrieve, and delete data using Django's low-level cache API.

Think about the basic cache operations: save, read, and remove.

Describe the difference between cache.add() and cache.set() in Django's low-level cache API.

Consider when you want to avoid overwriting existing cache data.

Practice

1. What is the main purpose of Django's low-level cache API?

easy

A. To store and retrieve data quickly to improve app speed

B. To manage database migrations automatically

C. To handle user authentication and permissions

D. To create HTML templates for web pages

Solution

1. Step 1: Understand the cache API role

   The low-level cache API is designed to store data temporarily for fast access.

2. Step 2: Compare with other options

   Other options relate to different Django features like migrations, auth, or templates, not caching.

3. Final Answer:

   To store and retrieve data quickly to improve app speed -> Option A

4. Quick Check:

   Cache API purpose = speed up data access [OK]

Hint: Cache API is for fast data storage and retrieval [OK]

Common Mistakes:

• Confusing cache with database migrations
• Mixing cache with authentication features
• Thinking cache creates HTML templates

2. Which of the following is the correct way to save a value in Django's low-level cache?

easy

A. cache.set('key', 'value')

B. cache.save('key', 'value')

C. cache.put('key', 'value')

D. cache.store('key', 'value')

Solution

1. Step 1: Recall the cache API method for saving

   The correct method to save data in Django cache is cache.set.

2. Step 2: Verify other options

   Methods like save, put, and store do not exist in Django's cache API.

3. Final Answer:

   cache.set('key', 'value') -> Option A

4. Quick Check:

   Use cache.set to save data [OK]

Hint: Use cache.set to save data in cache [OK]

Common Mistakes:

• Using cache.save instead of cache.set
• Confusing cache methods with other APIs
• Assuming put or store exist in cache API

3. What will be the output of this code snippet?

from django.core.cache import cache
cache.set('count', 5)
value = cache.get('count')
print(value)

medium

A. 0

B. None

C. KeyError

D. 5

Solution

1. Step 1: Analyze cache.set usage

   The code sets the key 'count' with value 5 in the cache.

2. Step 2: Analyze cache.get usage

   Retrieving 'count' returns the stored value 5, so print outputs 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   cache.get returns stored value 5 [OK]

Hint: cache.get returns stored value or None if missing [OK]

Common Mistakes:

• Expecting None if key was just set
• Thinking cache.get raises KeyError
• Confusing default return values

4. Identify the error in this code snippet using Django's low-level cache API:

from django.core.cache import cache
cache.set('user', 'Alice')
value = cache.get('user', 'Bob')
print(value)

medium

A. cache.get does not accept a default value

B. cache.set requires a timeout argument

C. No error; output will be 'Alice'

D. cache.get will raise an exception if key exists

Solution

1. Step 1: Check cache.set usage

   cache.set with key and value is valid; timeout is optional.

2. Step 2: Check cache.get with default

   cache.get accepts a default value returned if key is missing; here key exists, so returns 'Alice'.

3. Final Answer:

   No error; output will be 'Alice' -> Option C

4. Quick Check:

   cache.get returns stored value if key exists [OK]

Hint: cache.get default used only if key missing [OK]

Common Mistakes:

• Thinking cache.get can't take default
• Believing timeout is mandatory for cache.set
• Assuming cache.get raises error if key exists

5. You want to cache a user's profile data for 10 minutes using Django's low-level cache API. Which code snippet correctly does this?

hard

A. cache.set('profile_1', user_profile, expire=600)

B. cache.set('profile_1', user_profile, timeout=600)

C. cache.save('profile_1', user_profile, timeout=600)

D. cache.set('profile_1', user_profile, time=600)

Solution

1. Step 1: Identify correct timeout argument

   The cache.set method uses the argument timeout to specify cache duration in seconds.

2. Step 2: Check method and argument names

   Only cache.set('profile_1', user_profile, timeout=600) uses cache.set with timeout=600. Others use wrong method or argument names.

3. Final Answer:

   cache.set('profile_1', user_profile, timeout=600) -> Option B

4. Quick Check:

   Use cache.set with timeout for timed caching [OK]

Hint: Use timeout parameter in cache.set for expiry [OK]

Common Mistakes:

• Using expire or time instead of timeout
• Using cache.save which doesn't exist
• Omitting timeout for temporary cache