Explanation of this Python language feature? [x for x in x for x in x] (to flatten a nested list)

[Chris Angelico]

On Tue, Mar 25, 2014 at 1:04 AM, Steven D'Aprano
<steve+comp.lang.python at pearwood.info> wrote:
>> But which of these is truly more readable?
>>
>> squares = []
>> for n in range(30):
>>     squares.append(n * n)
>>
>> squares = [n * n for n in range(30)]
>
> Readable for whom?
>
> List comprehension syntax is often completely obscure to beginners. A
> beginner would say that the explicit for-loop is more readable.
>
> Actually, a *real* beginner, whose main programming experience before
> Python was Pascal, would probably even say that the first example was an
> unreadable mess. What's range(30)? What's this ".append" business? What
> does [] mean? I know this because I was this beginner, once. The first
> few times I tried reading Python code, I couldn't make head or tail of
> it. "for" I recognised, because it was the same keyword as Pascal and
> Hypertalk use. Pretty much everything else might as well have been
> Bulgarian.

Actually, that's a very good point. Python's for loop is more often
called a foreach loop in other languages, and Python completely lacks
any concept of a "classic" iteration-over-integer for loop. That is a
point of confusion. However, that's going to come up on both branches,
so it's not really a mark against either.

Incidentally, I've often modified my loop counter, in C or REXX or any
other language. About the only situation where I actually miss it in
Python, though, is iterating over a list and mutating the list on the
way through; and even that can often be done in other ways (maybe a
list comp, filtering out some of the elements?). It's amazing how
something can be so utterly fundamental (I mean, come ON! Who can
imagine a language with no equivalent of the basic "do i=1 to 10"
(REXX) or "for (int i=0;i<10;++i)" (C++) loop???) and yet so
dispensable.

ChrisA