Debajit Adhikary <[EMAIL PROTECTED]> wrote: ... > How does "a.extend(b)" compare with "a += b" when it comes to > performance? Does a + b create a completely new list that it assigns > back to a? If so, a.extend(b) would seem to be faster. How could I > verify things like these?
That's what the timeit module is for, but make sure that the snippet you're timing has no side effects (since it's repeatedly executed). E.g.: brain:~ alex$ python -mtimeit -s'z=[1,2,3];b=[4,5,6]' 'a=z[:];a.extend(b)' 1000000 loops, best of 3: 0.769 usec per loop brain:~ alex$ python -mtimeit -s'z=[1,2,3];b=[4,5,6]' 'a=z[:];a+=b' 1000000 loops, best of 3: 0.664 usec per loop brain:~ alex$ python -mtimeit -s'z=[1,2,3];b=[4,5,6]' 'a=z[:];a.extend(b)' 1000000 loops, best of 3: 0.769 usec per loop brain:~ alex$ python -mtimeit -s'z=[1,2,3];b=[4,5,6]' 'a=z[:];a+=b' 1000000 loops, best of 3: 0.665 usec per loop brain:~ alex$ The repetition of the measurements show them very steady, so now you know that += is about 100 nanoseconds faster (on my laptop) than extend (the reason is: it saves the tiny cost of looking up 'extend' on a; to verify this, use much longer lists and you'll notice that while overall times for both approaches increase, the difference between the two approaches remains about the same for lists of any length). But the key point to retain is: make sure that the snippet is free of side effects, so that each of the MANY repetitions that timeit does is repeating the SAME operation. If we initialized a in the -s and then just extended it in the snippet, we'd be extending a list that keeps growing at each repetition -- a very different operation than extending a list of a certain fixed starting length (here, serendipitously, we'd end up measuring the same difference -- but in the general case, where timing difference between approaches DOES depend on the sizes of the objects involved, our measurements would instead become meaningless). Therefore, we initialize in -s an auxiliary list, and copy it in the snippet. That's better than the more natural alternative: brain:~ alex$ python -mtimeit 'a=[1,2,3];a+=[4,5,6]' 1000000 loops, best of 3: 1.01 usec per loop brain:~ alex$ python -mtimeit 'a=[1,2,3];a.extend([4,5,6])' 1000000 loops, best of 3: 1.12 usec per loop brain:~ alex$ python -mtimeit 'a=[1,2,3];a+=[4,5,6]' 1000000 loops, best of 3: 1.02 usec per loop brain:~ alex$ python -mtimeit 'a=[1,2,3];a.extend([4,5,6])' 1000000 loops, best of 3: 1.12 usec per loop as in this "more natural alternative" we're also paying each time through the snippet the cost of building the literal lists; this overhead (which is a lot larger than the difference we're trying to measure!) does not DISTORT the measurement but it sure OBSCURES it to some extend (losing us about one significant digit worth of difference in this case). Remember, the WORST simple operation you can do in measurement is gauging a small number delta as the difference of two much larger numbers X and X+delta... so, make X as small as feasible to reduce the resulting loss of precision!-) You can find more details on commandline use of timeit at <http://docs.python.org/lib/node808.html> (see adjacent nodes in Python docs for examples and details on the more advanced use of timeit inside your own code) but I hope these indications may be of help anyway. Alex -- http://mail.python.org/mailman/listinfo/python-list
