Why list comprehension faster than for loop?

[Trent Nelson]

On 9 May 2010, at 16:29, Xavier Ho wrote:

On Sun, May 9, 2010 at 6:20 PM, gopi krishna <dasarathulagopi at gmail.com<mailto:dasarathulagopi at gmail.com>> wrote:
Why list comprehension faster than for loop?

Because Python optimises for certain special cases, when the number of iterations is predicable in a list comprehension.

The fact that list comprehensions can only be comprised of expressions, not statements, is a considerable factor, as much less work is required behind the scenes for each iteration.  Another factor is that the underlying iteration mechanism for list comprehensions is much closer to a C loop than execution of a for loop.  Exact same reason Perl's map { } is much faster than an equivalent for/foreach loop.  (Psyco can speed up certain list comprehensions quite significantly, too, but it isn't able to perform the same magic on an equivalent for loop.)

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