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python/snippets/intersection-by.md

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+---
+title: List intersection based on function
+type: snippet
+tags: [list,function]
+cover: duck-plants
+dateModified: 2020-11-02T19:28:05+02:00
+---
+
+Returns a list of elements that exist in both 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 contained in both lists.
+
+```py
+def intersection_by(a, b, fn):
+  _b = set(map(fn, b))
+  return [item for item in a if fn(item) in _b]
+```
+
+```py
+from math import floor
+
+intersection_by([2.1, 1.2], [2.3, 3.4], floor) # [2.1]
+```