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 6:13 AM, Mark Lawrence <breamoreboy at yahoo.co.uk> wrote:
>>     That was "a" proposal for 3000. Its there, but its not on the
>> built-ins; ie., you have to import it.  The confusion: why reduce, why
>> not filter, nor map?  {rhetorical}
>
>
> So it is in the standard library then.  And I'm not confused, seeing this
> must have been decided years ago as Python 3 was released some five years
> ago.

Terminology issue, is all. It's not in the builtins, but it is in the
standard library.

ChrisA