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copied!<p><strong>EDIT</strong>: Use <code>itertools.islice</code>. The pattern below that I originally proposed is bad idea — it crashes when <code>it</code> yields less than <code>n</code> values, and this behaviour depends on subtle issues, so people reading such code are unlikely to understand it's precise semantics.</p> <blockquote> <p>There is also</p> <pre><code>[next(it) for _ in range(n)] </code></pre> <p>which might(?) be clearer to people not familiar with itertools; but if you deal with iterators a lot, itertools is a worthy addition to your toolset.</p> </blockquote> <h3>What happens if <code>next(it)</code> was exhausted and raises <code>StopIteration</code>?</h3> <p>(i.e. when <code>it</code> had less than <code>n</code> values to yield)</p> <p>When I wrote the above line a couple years ago, I probably thought a <code>StopIteration</code> will have the clever side effect of cleanly terminating the list comprehension. But no, the whole comprehension will crash passing the <code>StopIteration</code> upwards. (It'd exit cleanly only if the exception originated from the <code>range(n)</code> iterator.)</p> <p>Which is probably not the behavior you want.</p> <p>But it gets worse. The following is supposed to be equivalent to the list comprehension (especially on Python 3):</p> <pre><code>list(next(it) for _ in range(n)) </code></pre> <p>It isn't. The inner part is shorthand for a generator function; <code>list()</code> knows it's done when it raises <code>StopIteration</code> <em>anywhere</em>.<br> => This version copes safely when there aren't <code>n</code> values and returns a shorter list. (Like <code>itertools.islice()</code>.)</p> <p>[Executions on: <a href="https://ideone.com/KJeZii" rel="nofollow noreferrer">2.7</a>, <a href="https://ideone.com/KJeZii" rel="nofollow noreferrer">3.4</a>]</p> <p>But that's too going to change! The fact a generator silently exits when any code inside it raises <code>StopIteration</code> is a known wart, addressed by <a href="https://www.python.org/dev/peps/pep-0479/" rel="nofollow noreferrer">PEP 479</a>. From Python 3.7 (or 3.5 with a future import) that's going to cause a <code>RuntimeError</code> instead of cleanly finishing the generator. I.e. it'll become similar to the list comprehension's behaviour. (Tested on a recent HEAD build)</p>

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