Start of main content## More like this

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

- Create a
`set`

, using`map()`

to apply`fn`

to each element in`b`

. - Use a list comprehension in combination with
`fn`

on`a`

to only keep values contained in both lists.

```
def intersection_by(a, b, fn):
_b = set(map(fn, b))
return [item for item in a if fn(item) in _b]
```

```
from math import floor
intersection_by([2.1, 1.2], [2.3, 3.4], floor) # [2.1]
```

### List union based on function

Python, List

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

### 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 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.