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    <p>Here are three possibilities:</p> <pre><code>foo = """ this is a multi-line string. """ def f1(foo=foo): return iter(foo.splitlines()) def f2(foo=foo): retval = '' for char in foo: retval += char if not char == '\n' else '' if char == '\n': yield retval retval = '' if retval: yield retval def f3(foo=foo): prevnl = -1 while True: nextnl = foo.find('\n', prevnl + 1) if nextnl &lt; 0: break yield foo[prevnl + 1:nextnl] prevnl = nextnl if __name__ == '__main__': for f in f1, f2, f3: print list(f()) </code></pre> <p>Running this as the main script confirms the three functions are equivalent. With <code>timeit</code> (and a <code>* 100</code> for <code>foo</code> to get substantial strings for more precise measurement):</p> <pre><code>$ python -mtimeit -s'import asp' 'list(asp.f3())' 1000 loops, best of 3: 370 usec per loop $ python -mtimeit -s'import asp' 'list(asp.f2())' 1000 loops, best of 3: 1.36 msec per loop $ python -mtimeit -s'import asp' 'list(asp.f1())' 10000 loops, best of 3: 61.5 usec per loop </code></pre> <p>Note we need the <code>list()</code> call to ensure the iterators are traversed, not just built.</p> <p>IOW, the naive implementation is so much faster it isn't even funny: 6 times faster than my attempt with <code>find</code> calls, which in turn is 4 times faster than a lower-level approach.</p> <p>Lessons to retain: measurement is always a good thing (but must be accurate); string methods like <code>splitlines</code> are implemented in very fast ways; putting strings together by programming at a very low level (esp. by loops of <code>+=</code> of very small pieces) can be quite slow.</p> <p><strong>Edit</strong>: added @Jacob's proposal, slightly modified to give the same results as the others (trailing blanks on a line are kept), i.e.:</p> <pre><code>from cStringIO import StringIO def f4(foo=foo): stri = StringIO(foo) while True: nl = stri.readline() if nl != '': yield nl.strip('\n') else: raise StopIteration </code></pre> <p>Measuring gives:</p> <pre><code>$ python -mtimeit -s'import asp' 'list(asp.f4())' 1000 loops, best of 3: 406 usec per loop </code></pre> <p>not quite as good as the <code>.find</code> based approach -- still, worth keeping in mind because it might be less prone to small off-by-one bugs (any loop where you see occurrences of +1 and -1, like my <code>f3</code> above, should automatically trigger off-by-one suspicions -- and so should many loops which lack such tweaks and should have them -- though I believe my code is also right since I was able to check its output with other functions').</p> <p>But the split-based approach still rules.</p> <p>An aside: possibly better style for <code>f4</code> would be:</p> <pre><code>from cStringIO import StringIO def f4(foo=foo): stri = StringIO(foo) while True: nl = stri.readline() if nl == '': break yield nl.strip('\n') </code></pre> <p>at least, it's a bit less verbose. The need to strip trailing <code>\n</code>s unfortunately prohibits the clearer and faster replacement of the <code>while</code> loop with <code>return iter(stri)</code> (the <code>iter</code> part whereof is redundant in modern versions of Python, I believe since 2.3 or 2.4, but it's also innocuous). Maybe worth trying, also:</p> <pre><code> return itertools.imap(lambda s: s.strip('\n'), stri) </code></pre> <p>or variations thereof -- but I'm stopping here since it's pretty much a theoretical exercise wrt the <code>strip</code> based, simplest and fastest, one.</p>
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    1. COAlso, `(line[:-1] for line in cStringIO.StringIO(foo))` is pretty fast; almost as fast as the naive implementation, but not quite.
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      1. USMatt Anderson
    2. COThank you for this great answer. I guess the main lesson here (as I am new to python) is to make using `timeit` a habit.
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      1. USBjörn Pollex
    3. CO@Space, yep, timeit is good, any time you care about performance (be sure to use it carefully, e.g. in this case see my note about needing a `list` call to actually time all the relevant parts!-).
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