Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.

def union_by(a, b, fn):
 _a = set(map(fn, a))
 return list(set(a + [item for item in b if fn(item) not in _a]))

from math import floor

union_by([2.1], [1.2, 2.3], floor) # [2.1, 1.2]

Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.
<ul>
<li>Create a <code class="notranslate">set</code> by applying <code class="notranslate">fn</code> to each element in <code class="notranslate">a</code>.</li>
<li>Use a list comprehension in combination with <code class="notranslate">fn</code> on <code class="notranslate">b</code> to only keep values not contained in the previously created set, <code class="notranslate">_a</code>.</li>
<li>Finally, create a <code class="notranslate">set</code> from the previous result and <code class="notranslate">a</code> and transform it into a <code class="notranslate">list</code></li>
</ul>

List union based on function

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