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snippets/python/s/symmetric-difference-by.md

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+---
+title: List symmetric difference based on function
+type: snippet
+language: python
+tags: [list]
+cover: succulent-1
+dateModified: 2020-11-02T19:28:35+02:00
+---
+
+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.
+
+```py
+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]
+```
+
+```py
+from math import floor
+
+symmetric_difference_by([2.1, 1.2], [2.3, 3.4], floor) # [1.2, 3.4]
+```