Start of main content## Recommended snippets

Returns the difference between two 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 not contained in the previously created set,`_b`

.

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

```
from math import floor
difference_by([2.1, 1.2], [2.3, 3.4], floor) # [1.2]
difference_by([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], lambda v : v['x'])
# [ { x: 2 } ]
```

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### 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.

### 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 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.