On Fri, 20 Aug 2010 09:51:08 am Pete wrote:
[...]
> Ah, so list comprehensions are a purely syntactic construct?
No, I don't think that's what Emile was trying to say. It's not like
list comps are macros that are expanded into the explicit for-loop like
that. All he is saying is that both the for-loop and the list
comprehension loop over the list.
There are some functional differences: for example, the list comp avoids
needing to look up result.append over and over again. Because it knows
it is dealing with a list, rather than some arbitrary object that
conceivably might have some weird append method with strange
side-effects, it can take optimizing short-cuts that aren't applicable
to general for-loops.
> I had the feeling there might be a performance benefit of some kind.
If you write the for-loop in the most straight forward, obvious fashion,
then there is a small but measurable performance cost from repeatedly
looking up the append method. On the other hand, for-loops are more
general -- you can call break to exit early, or continue to skip an
iteration, so they're easier to optimize. There's no way to break out
of a list comp early (other than raising an exception, which defeats
the purpose). No matter how fast you can perform a loop, it's always
faster to avoid it altogether, so this:
seq = xrange(10000000)
result = []
for i in seq:
if i >= 10: break
result.append(i%2)
will be much faster than this:
seq = xrange(10000000)
result = [i%2 for i in seq if i < 10]
Other than that, the speed of the loop itself is virtually the same as
the speed of the list comprehension.
--
Steven D'Aprano
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