Redis ZUNION in Python (Detailed Guide w/ Code Examples)

Use Case(s)

The ZUNION command in Redis is used to compute the union of multiple sorted sets and store the resulting sorted set in a new key. It's particularly useful for aggregating scores from different datasets, such as combining user rankings from various aspects (e.g., activity level, purchase volume) into a single score.

Code Examples

Basic Usage of ZUNION

In this example, we will calculate the union of two sorted sets and store the result in a new sorted set using ZUNION.

import redis # Connect to Redis client = redis.Redis(host='localhost', port=6379, db=0) # Add some elements to the sorted sets client.zadd('sorted_set_1', {'Alice': 1, 'Bob': 2}) client.zadd('sorted_set_2', {'Alice': 3, 'Charlie': 2}) # Compute the union and store it in a new sorted set result_key = 'combined_set' client.zunionstore(dest=result_key, keys=['sorted_set_1', 'sorted_set_2'], aggregate='SUM') # Fetch and print the combined sorted set print(client.zrange(result_key, 0, -1, withscores=True))

This script connects to Redis, creates two sorted sets, calculates their union using ZUNIONSTORE, and stores the result in a new sorted set. The results are then retrieved and displayed.

Using Weights with ZUNION

You can also apply different weights to the sorted sets during the union operation. This is useful when the scores in different sets have different scales or importance.

# Apply weights to the inputs; double the influence of the first sorted set client.zunionstore(dest=result_key, keys=['sorted_set_1', 'sorted_set_2'], weights=[2, 1], aggregate='SUM') # Fetch and print the weighted combined sorted set print(client.zrange(result_key, 0, -1, withscores=True))

In this example, scores from sorted_set_1 are considered twice as important as those from sorted_set_2 by specifying weights.

Best Practices

  • Use Appropriate Aggregation: By default, scores are summed. However, based on your application needs, you might want to use MIN or MAX to get the minimum or maximum score respectively.
  • Memory Management: Remember to delete any temporary or unneeded keys to avoid unnecessary memory usage.

Common Mistakes

  • Overlooking Return Values: Always check the return value of zunionstore. It returns the number of elements in the resulting set, which can be useful for error handling and verification.
  • Ignoring Connection Issues: Ensure robust connection handling and error checking especially in production environments.


Q: What happens if one of the sorted sets does not exist? A: Redis treats non-existing keys as empty sets. Thus, if only one of the keys exists, the result will be the same as the existing set.

Q: Can ZUNION operate directly on more than two sorted sets? A: Yes, ZUNION can combine any number of sorted sets as specified in the keys argument.

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