Low-level cache API in Django - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Django cache code?

Consider this Django code snippet using the low-level cache API:

from django.core.cache import cache

cache.set('key1', 'value1', timeout=5)
value = cache.get('key1')
print(value)

What will be printed immediately after running this code?

Django

from django.core.cache import cache

cache.set('key1', 'value1', timeout=5)
value = cache.get('key1')
print(value)

A value1

B None

C KeyError

D TimeoutError

💡 Hint

Think about what cache.set and cache.get do immediately after setting a value.

The cache.set stores 'value1' under 'key1' with a 5-second timeout. Immediately after, cache.get('key1') returns 'value1' because the key exists and has not expired.

❓ state_output

intermediate

What is the value of 'result' after this cache operation?

Given this code snippet:

from django.core.cache import cache

cache.set('counter', 10)
result = cache.incr('counter', 5)

What is the value of result after running this?

Django

from django.core.cache import cache

cache.set('counter', 10)
result = cache.incr('counter', 5)

A None

B 15

C 10

D ValueError

💡 Hint

What does cache.incr do to the stored value?

cache.incr increments the integer value stored under 'counter' by 5, so 10 + 5 = 15.

📝 Syntax

advanced

Which option causes a syntax error in Django cache usage?

Identify which code snippet will cause a syntax error when using Django's low-level cache API:

A cache.set('key', 'value', timeout=60)

B cache.get('key', default=None)

C cache.set('key', 'value' timeout=60)

D cache.delete('key')

💡 Hint

Look carefully at the commas separating arguments.

Option C is missing a comma between 'value' and timeout=60, causing a syntax error.

🔧 Debug

advanced

Why does this cache increment raise an error?

Given this code:

from django.core.cache import cache

cache.set('visits', 'ten')
cache.incr('visits')

What error will be raised and why?

Django

from django.core.cache import cache

cache.set('visits', 'ten')
cache.incr('visits')

A ValueError because 'ten' is not an integer

B KeyError because 'visits' does not exist

C TypeError because incr expects a string

D No error, increments to 'ten1'

💡 Hint

cache.incr expects the stored value to be an integer.

cache.incr tries to add 1 to the stored value. Since 'ten' is a string, it raises ValueError.

🧠 Conceptual

expert

Which statement about Django's low-level cache API is TRUE?

Select the correct statement about Django's low-level cache API behavior:

A cache.delete() raises an error if the key does not exist

B cache.set() automatically serializes complex Python objects without configuration

C cache.incr() can increment keys that do not exist by creating them with value 1

D cache.get() returns None if the key does not exist and no default is provided

💡 Hint

Think about default return values and error handling in cache methods.

cache.get() returns None if the key is missing and no default is given. cache.set() requires objects to be serializable. cache.incr() raises a ValueError if the key does not exist. cache.delete() does not raise an error if the key is missing.

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