The Cost of Dynamism (was Re: Pyhon 2.x or 3.x, which is faster?)

[Ned Batchelder]

On Tuesday, March 22, 2016 at 7:05:20 AM UTC-4, BartC wrote:
> On 22/03/2016 01:01, Steven D'Aprano wrote:
> > Pythonic code probably uses a lot of iterables:
> >
> > for value in something:
> >      ...
> 
> > in preference to Pascal code written in Python:
> >
> > for index in range(len(something)):
> >      value = something[index]
> 
> (Suppose you need both the value and its index in the loop? Then the 
> one-line for above won't work. For example, 'something' is [10,20,30] 
> and you want to print:
> 
>   0: 10
>   1: 20
>   2: 30 )

Then you use enumerate:

    for index, value in enumerate(something):
        print("{}: {}".format(index, value))

Python has a number of iteration features that you don't find in other
languages.  You might find this introduction to them helpful:

Loop Like a Native: http://nedbatchelder.com/text/iter.html

> 
> >      ...
> > or worse:
> >
> > index = 0
> > while index < len(something):
> >      value = something[index]
> >      ...
> >      index += 1
> 
> > (I don't know where that while-loop idiom comes from. C? Assembly? Penitent
> > monks living in hair shirts in the desert and flogging themselves with
> > chains every single night to mortify the accursed flesh? But I'm seeing it
> > a lot in code written by beginners. I presume somebody, or some book, is
> > teaching it to them. "Learn Python The Hard Way" perhaps?)
> 
> Are you suggesting 'while' is not needed? Not everything fits into a 
> for-loop you know!

Steven wasn't saying 'while' is not needed. He was wondering about the
idiom of manually maintaining an integer count of the number of times
around a while loop ("I don't know where *that* while-loop idiom comes from").

While-loops are still useful in Python, but for-loops are much more common.

--Ned.