---
title: union_by
tags: list,function,intermediate
---

Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.

Create a `set` by applying `fn` to each element in `a`, then use list comprehension in combination with `fn` on `b` to only keep values not contained in the previously created set, `_a`.
Finally, create a `set` from the previous result and `a` and transform it into a `list`

```py
def union_by(a,b,fn):
  _a = set(map(fn, a))
  return list(set(a + [item for item in b if fn(item) not in _a]))
```

```py
from math import floor
union_by([2.1], [1.2, 2.3], floor) # [2.1, 1.2]
```