Troubleshooting Wrong Type Error in Python Redis GET (Detailed Guide w/ Code Examples)

Use Case(s)

Encountering a WRONGTYPE error while using Python Redis GET typically happens when trying to perform an operation that is incompatible with the data type of the stored value. This issue often occurs when using data structures like lists, sets, or hashes.

Code Examples

Let's look at two examples where we try to fetch a list and a set as if they were strings - this will throw WRONGTYPE errors.

Example 1 - Fetching a List

import redis
r = redis.Redis()
r.rpush('mylist', 'item')  # Add item into a list
print(r.get('mylist'))  # Attempt to get the list as a string

This code raises a redis.exceptions.ResponseError: WRONGTYPE Operation against a key holding the wrong kind of value because we're trying to get a list as if it was a string.

Example 2 - Fetching a Set

import redis
r = redis.Redis()
r.sadd('myset', 'item')  # Add item into a set
print(r.get('myset'))  # Attempt to get the set as a string

This code also raises a WRONGTYPE error for similar reasons.

Best Practices

Ensure you're using the correct commands associated with the particular data type of the stored value. For example, use LRANGE to retrieve lists, SMEMBERS for sets, and so on.

Common Mistakes

A common mistake leading to the WRONGTYPE error is not understanding the types of data structures in Redis and their associated commands. Python's redis module adheres closely to the command structure of Redis itself, so knowledge of Redis data types is crucial.

FAQs

Q: How do I know which type of value a key holds? A: You can use the TYPE command. If you're unsure what type of data a key holds, you can run r.type('mykey'), which will return the type of the key.