n_unique
Joram Mutenge
2025-10-01
Knowing the cardinality of columns in your table is important. What’s cardinality? It’s the number of unique values contained in a column. If a column has many unique values, like Customer_ID, then it has high cardinality. Fewer unique values mean low cardinality. Below is a dataframe showing student grades.
| Student | Grade |
|---|---|
| str | i64 |
| "Spencer" | 93 |
| "Spencer" | 88 |
| "Emily" | 73 |
| "Emily" | 69 |
| "Hannah" | 55 |
| … | … |
| "Hannah" | 89 |
| "Aria" | 92 |
| "Aria" | 81 |
| "Mona" | 97 |
| "Paige" | 71 |
To get the number of unique values in a column, you use the Polars method n_unique like this:
The Student column has 6 unique values, which means it has low cardinality.
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