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python/snippets/difference-by.md

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+---
+title: List difference based on function
+type: snippet
+tags: [list,function]
+cover: coconuts
+dateModified: 2020-11-02T19:27:53+02:00
+---
+
+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`.
+
+```py
+def difference_by(a, b, fn):
+  _b = set(map(fn, b))
+  return [item for item in a if fn(item) not in _b]
+```
+
+```py
+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 } ]
+```