# Redis ZUNIONSTORE in Python (Detailed Guide w/ Code Examples)

## Use Case(s)

The `ZUNIONSTORE`

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 in scenarios such as combining scores from different games into a single leaderboard, or aggregating results from various metrics that have a notion of ranking or scoring.

## Code Examples

### Basic Example of ZUNIONSTORE

In this example, we use `ZUNIONSTORE`

to combine scores from two game leaderboards into a combined leaderboard.

`import redis r = redis.Redis(host='localhost', port=6379, db=0) # Assuming 'game1_scores' and 'game2_scores' are existing sorted sets result = r.zunionstore('combined_scores', ['game1_scores', 'game2_scores']) print(f"Number of elements in 'combined_scores': {result}")`

This code initializes a connection to Redis, then combines 'game1_scores' and 'game2_scores' into a new sorted set called 'combined_scores'.

### Using Weights and Aggregate Options

You can also apply weights to the scores from different sets and choose an aggregation method (sum, min, or max).

`result = r.zunionstore('weighted_combined_scores', {'game1_scores': 1, 'game2_scores': 2}, aggregate='max') print(f"Number of elements in 'weighted_combined_scores': {result}")`

This example creates a combined score list where scores from 'game2_scores' are twice as significant as 'game1_scores'. The maximum score between the two for each player is stored using the `aggregate='max'`

option.

## Best Practices

- When using
`ZUNIDONSTORE`

, carefully consider the memory usage, especially when dealing with large number of keys or very large sorted sets. - Ensure that the data types of the keys involved are all sorted sets. Attempting to use
`ZUNIONSTORE`

with other data types will result in an error.

## Common Mistakes

- Forgetting to handle the scenario where one or more keys do not exist. Redis treats them as empty sets, but depending on your application logic, this might need special handling.
- Not specifying weights correctly: Weights must be numbers and applied consistently if you are trying to achieve a weighted union.

## FAQs

### What happens if one of the sorted sets does not exist?

Redis treats non-existing keys as empty sorted sets during the operation.

### How does Redis handle ties in scores within sorted sets?

Redis ranks items with the same score by lexicographical order of their members.

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