Imagine two class teachers each keep a register of students who signed up for the annual fest. Some names appear only in the first register, some only in the second, and a few enthusiastic students signed up with both teachers. Now the principal asks for one combined list of everyone who signed up. You would copy out all the names from both registers — but you would write each student's name only once, because listing someone twice does not make them two people. That combined, duplicate-free list is exactly what a union is.

The union of two collections is a new collection containing every item that appears in the first, in the second, or in both. Two things make it different from simply stapling the two lists together:

• Every distinct item from both sides is included — nothing that belongs to either collection is left out.
• No item is repeated. If something appears in both collections, the union carries it just once.

This idea shows up throughout Informatics Practices wherever data is treated as a set of distinct values. In Python, sets support a union operation directly: given two sets, a.union(b) (or the pipe operator written as a | b) produces a new set holding every element found in either one, with duplicates automatically removed — that is simply what a set is. In databases, the same idea appears when you combine the results of two similar queries into one result: rows that satisfy either condition appear together, and identical rows are not repeated. …