# List symmetric difference based on function

Python, List · Nov 2, 2020

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

- Create a
`set`

by applying`fn`

to each element in every list. - Use a list comprehension in combination with
`fn`

on each of them to only keep values not contained in the previously created set of the other.

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

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