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    <p>A true <code>itertools</code>-based solution that works on an iterable:</p> <pre><code>&gt;&gt;&gt; part_iter = iter(part_list) &gt;&gt;&gt; part_in, part_out = itertools.tee(part_iter) &gt;&gt;&gt; in_main = (p for p in part_in if p in main_list) &gt;&gt;&gt; out_main = (p for p in part_out if p not in main_list) </code></pre> <p>Making lists out of these defeats the point of using iterators, but here is the result:</p> <pre><code>&gt;&gt;&gt; list(in_main) [4, 1] &gt;&gt;&gt; list(out_main) [5, 2, 7] </code></pre> <p>This has the advantage of lazily generating <code>in_main</code> and <code>out_main</code> from another lazily generated sequence. The only catch is that if you iterate through one before the other, <code>tee</code> has to cache a bunch of data until it's used by the other iterator. So this is really only useful if you iterate through them both at roughly the same time. Otherwise you might as well use auxiliary storage yourself.</p> <p>There's also an interesting ternary operator-based solution. (You could squish this into a list comprehension, but that would be wrong.) I changed main_list into a set for O(1) lookup. </p> <pre><code>&gt;&gt;&gt; main_set = set(main_list) &gt;&gt;&gt; in_main = [] &gt;&gt;&gt; out_main = [] &gt;&gt;&gt; for p in part_list: ... (in_main if p in main_set else out_main).append(p) ... &gt;&gt;&gt; in_main [4, 1] &gt;&gt;&gt; out_main [5, 2, 7] </code></pre> <p>There's also a fun <code>collections.defaultdict</code> approach:</p> <pre><code>&gt;&gt;&gt; import collections &gt;&gt;&gt; in_out = collections.defaultdict(list) &gt;&gt;&gt; for p in part_list: ... in_out[p in main_list].append(p) ... &gt;&gt;&gt; in_out defaultdict(&lt;type 'list'&gt;, {False: [5, 2, 7], True: [4, 1]}) </code></pre>
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    1. POPython List comprehension to create unequal length lists from a list using conditional
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    1. CO+1 for the second solution but not sure about the itertools solution.
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    2. CO@jamylak, not sure why?
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    3. COSeems like it is over-doing it, it would also involve zipping the final result and filtering the `None` out of the `in_main` and `out_main`.
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