# List difference based on function

OSCC · Python, List, Function · Nov 2, 2020

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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