Start of main content## More like this

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`

. - Use a 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`

```
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]))
```

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

### List difference based on function

Python, List

Returns the difference between two lists, after applying the provided function to each list element of both.

### List intersection based on function

Python, List

Returns a list of elements that exist in both lists, after applying the provided function to each list element of both.

### List symmetric difference based on function

Python, List

Returns the symmetric difference between two lists, after applying the provided function to each list element of both.