[Tutor] permutations?

[Francesco Loffredo]

On 03/12/2010 1.32, Steven D'Aprano wrote:
> Back in Ancient Days when dinosaurs walked the earth, and I programmed in Pascal, computers didn't have much memory, and were slow.
> Consequently it wasn't practical to make a copy of a list if you wanted to delete a few items. The only practical way to modify
> lists was to modify them in place, and if you needed to delete items, you had to work backwards. It took me a while to break myself
> of the habit of doing this:
>
> for i in range(len(mylist)-1, -1, -1):
>      if mylist[i] == "something":
>           del mylist[i]
>
> (Note that you only need to work backwards if you're *deleting* entries, not if you replace them with something else.)
>
> This is still a useful technique to have in your toolbox, but generally speaking the above is better written as:
>
> mylist = [x for x in mylist if x != "something"]
Up to this point, I share experiences and solution. But the next point did thrill me:
>
> If you really need to modify the list in place, and not just re-bind the name "mylist" to the new list, then one tiny change will do
> it:
>
> mylist[:] = [x for x in mylist if x != "something"]
>
I thought mylist[:] was a copy of mylist, and the two lines above would generate exactly the same code!

Is this a special optimization by the Python interpreter, or is it just a mistake in my understanding of the slice operator?


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