Redis with Python (redis-py) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this Redis-py command?

Given a Redis instance with a key 'counter' set to '10', what will be the output of the following Python code?

import redis
r = redis.Redis()
result = r.incr('counter')
print(result)

Redis

import redis
r = redis.Redis()
result = r.incr('counter')
print(result)

A 11

B 10

C TypeError

D None

💡 Hint

The INCR command increases the integer value stored at key by one and returns the new value.

The INCR command increments the value stored at 'counter' by 1. Since it was 10, the new value is 11, which is returned and printed.

📝 Syntax

intermediate

Which option correctly sets a key with expiration using redis-py?

You want to set a key 'session' with value 'abc123' that expires after 60 seconds. Which code snippet is correct?

A r.set('session', 'abc123', ex=60)

B r.set('session', 'abc123', expire=60)

C r.set('session', 'abc123', timeout=60)

D r.set('session', 'abc123', ttl=60)

💡 Hint

Check the redis-py documentation for the parameter name to set expiration in seconds.

The correct parameter to set expiration time in seconds in redis-py set() method is 'ex'. Other options are invalid and cause errors.

❓ optimization

advanced

How to efficiently retrieve multiple keys in redis-py?

You need to get the values of keys 'key1', 'key2', and 'key3' from Redis. Which approach is the most efficient?

A for k in ['key1', 'key2', 'key3']: r.get(k)

B r.get('key1'), r.get('key2'), r.get('key3')

C r.mget(['key1', 'key2', 'key3'])

D r.pipeline().get('key1').get('key2').get('key3').execute()

💡 Hint

Look for a redis-py command that fetches multiple keys in one call.

The mget() method fetches multiple keys in a single call, reducing network overhead. Option C uses pipeline but is more complex and less direct.

🔧 Debug

advanced

Why does this redis-py code raise a TypeError?

Consider this code snippet:

r.set('count', 5)
value = r.incr('count')
print(value)

Why does it raise a TypeError?

Redis

r.set('count', 5)
value = r.incr('count')
print(value)

A Because r.incr only works on keys that do not exist

B Because r.incr cannot increment a key set with a string

C Because r.set stores the integer as bytes, causing incr to fail

D Because r.set expects a string value, not an integer

💡 Hint

Check the data type of the value passed to r.set and what redis-py expects.

r.set expects the value to be bytes or string. Passing an integer directly causes a TypeError. You should convert the integer to string or bytes.

🧠 Conceptual

expert

What happens if you call r.get() on a non-existent key in redis-py?

If you run r.get('missing_key') on a Redis instance where 'missing_key' does not exist, what is the result?

Redis

r.get('missing_key')

A Raises a KeyError

B Returns None

C Returns an empty string ''

D Returns 0

💡 Hint

Think about how redis-py handles keys that are not found.

When a key does not exist, r.get returns None, indicating absence of value without error.

Practice

1. What does the set() method do in redis-py?

easy

A. Lists all keys stored in Redis

B. Retrieves a value by key from Redis

C. Deletes a key from Redis

D. Stores a value with a specified key in Redis

Solution

1. Step 1: Understand the purpose of set()

   The set() method is used to save data in Redis by associating a value with a key.

2. Step 2: Compare with other methods

   get() retrieves data, delete() removes keys, and keys() lists keys, so they do not store data.

3. Final Answer:

   Stores a value with a specified key in Redis -> Option D

4. Quick Check:

   set() means store data [OK]

Hint: Remember: set = save data, get = retrieve data [OK]

Common Mistakes:

• Confusing set() with get()
• Thinking set() deletes keys
• Assuming set() lists keys

2. Which of the following is the correct syntax to get a value for key 'name' using redis-py?

easy

A. r.get('name')

B. r.get(name)

C. r.get['name']

D. r.get{name}

Solution

1. Step 1: Identify correct method call syntax

   In Python, method calls use parentheses with arguments inside, so r.get('name') is correct.

2. Step 2: Check argument format

   The key must be a string, so it needs quotes. Options A, B, and D have syntax errors or missing quotes.

3. Final Answer:

   r.get('name') -> Option A

4. Quick Check:

   Method call with parentheses and string key [OK]

Hint: Use parentheses and quotes for string keys in method calls [OK]

Common Mistakes:

• Omitting quotes around string keys
• Using square brackets instead of parentheses
• Using curly braces instead of parentheses

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

import redis
r = redis.Redis()
r.set('color', 'blue')
value = r.get('color')
print(value)

medium

A. blue

B. b'blue'

C. None

D. Error

Solution

1. Step 1: Understand Redis returns bytes

   The get() method returns the value as bytes, so value is b'blue'.

2. Step 2: Print output

   Printing value shows the bytes literal b'blue', not the decoded string.

3. Final Answer:

   b'blue' -> Option B

4. Quick Check:

   Redis returns bytes, so output shows b'...' [OK]

Hint: Remember: Redis get returns bytes, not decoded string [OK]

Common Mistakes:

• Expecting a plain string without decoding
• Assuming None if key exists
• Thinking code raises error

4. Identify the error in this code snippet:

import redis
r = redis.Redis()
r.set('age', 30)
age = r.get('age')
print(age)

medium

A. get() returns bytes, need decoding

B. Cannot store integer directly with set()

C. Missing Redis server connection parameters

D. Syntax error in print statement

Solution

1. Step 1: Check data storage

   Redis-py converts integers to strings automatically, so storing 30 is valid.

2. Step 2: Check retrieval and printing

   get() returns bytes, so printing age shows bytes, not a readable string or integer. Decoding is needed.

3. Final Answer:

   get() returns bytes, need decoding -> Option A

4. Quick Check:

   Decode bytes after get() to read value [OK]

Hint: Always decode bytes from get() before printing [OK]

Common Mistakes:

• Thinking set() rejects integers
• Ignoring bytes return type from get()
• Assuming print syntax error

5. You want to store user info as a dictionary in Redis using redis-py. Which approach correctly saves and retrieves the dictionary?

hard

A. Store dictionary as string with set() and retrieve with get() without decoding

B. Directly use set() with the dictionary and get() to retrieve

C. Use set() with json.dumps() to save and json.loads() after get()

D. Use hmset() to save and hgetall() to retrieve

Solution

1. Step 1: Understand Redis stores strings/bytes

   Redis cannot store Python dicts directly; they must be converted to strings.

2. Step 2: Use JSON to serialize and deserialize

   Using json.dumps() converts dict to string for set(), and json.loads() converts back after get().

3. Step 3: Evaluate other options

   hmset() is deprecated; direct dict storage fails; storing string without decoding causes errors.

4. Final Answer:

   Use set() with json.dumps() and json.loads() after get() -> Option C

5. Quick Check:

   Serialize dict to string before storing, deserialize after retrieval [OK]

Hint: Serialize dict with JSON before set(), deserialize after get() [OK]

Common Mistakes:

• Trying to store dict directly
• Using deprecated hmset() method
• Not decoding bytes before json.loads()