# List union based on function

Python, List · Nov 2, 2020

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]```

## More like this

• ### List difference based on function

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

• ### List intersection based on function

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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

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