# List symmetric difference based on function

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]```

## More like this

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### List difference based on function

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

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### List union based on function

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

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### List intersection based on function

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

• Python ·

### List symmetric difference

Returns the symmetric difference between two iterables, without filtering out duplicate values.

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