PEP 8 : Maximum line Length :

[Chris Angelico]

On Fri, May 16, 2014 at 7:18 PM, Marko Rauhamaa <marko at pacujo.net> wrote:
> If every bit of your Python text conveys information, obviously, it
> can't be abstracted. I don't believe that to be the case, though. So
> this AST should contain all *actual* information worth conveying and
> strip away irrelevant stuff.
>
> Examples:
>
>  * Function definition order.
>
>  * Indentation depth.
>
>  * Vertical empty space.

* Logical layout as expressed by whitespace and specific line breaks.

Compare these two assignment statements:

area = (base*base + extension*extension
    + annex*annex + (annex-extension)*annex
    + triangle*triangle/2
    + circle*circle*math.PI + sphere*sphere*4*math.PI)

area = (base*base + extension*extension + annex*annex
    + (annex-extension)*annex + triangle*triangle/2
    + circle*circle*math.PI + sphere*sphere*4*math.PI)

Both are wrapped to sixty characters, which means they can be indented
twenty without breaking PEP 8. One of them takes up only three lines,
the other takes up four. But the first one groups the annex's terms,
separates out the triangle, groups the circular elements, and
presumably corresponds accurately to the (mythical) real-world
geometry that it's implementing. How are you going to cope with this
distinction? That's real, useful information, which the AST doesn't
carry.

(You might argue that each of these lines should have a comment at the
end of it or above it, which would provide that grouping. But in that
case, you lose the canonicalization of anything with trailing comments
or interspersed comments, meaning that you effectively can't reformat
anything with comments in it.)

ChrisA