"Collapsing" a list into a list of changes

[Peter Otten]

John Lenton wrote:

> On Mon, Feb 07, 2005 at 09:39:11PM +0100, Peter Otten wrote:
>> John Lenton wrote:
>> >                                         For example, the fastest way
>> > to get the factorial of a (small enough) number in pure python is
>> > 
>> >   factorial = lambda n: reduce(operator.mul, range(1, n+1))
>>  
>> I see that you are seduced by the beauty of the expression. Otherwise, if
>> you would really care for speed:
>> 
>> [...]
> 
> that's cheating: you moved the calculation into the setup. You aren't
> timing what you say you are.

*You* are cheating when you take a predefined function implemented in C
(operator.mul) and then claim you are using pure Python. This gives you the
extra 50 percent that overcompensate the inefficiency of reduce() -- but
only by a hair's breadth:

$ python2.4 -m timeit -s'mul = lambda x, y: x * y' -s'a=1' 'mul(a, a)'
1000000 loops, best of 3: 0.522 usec per loop
$ python2.4 -m timeit -s'from operator import mul' -s'a=1' 'mul(a, a)'
1000000 loops, best of 3: 0.345 usec per loop


Peter