[Python-ideas] Extending expressions using ellipsis

[Shane Hathaway]

On 09/01/2016 01:24 PM, Brendan Barnwell wrote:
>     I sometimes write similar code with a sequence of pandas
> operations.  I think you can get a lot of mileage out of accepting the
> loss in precious vertical space and just putting the closing parenthesis
> on its own line, indented to the same level as the beginning of the line
> where the corresponding open parenthesis was.  Your example then becomes:
>
> rows = (
>      dbsession.query(Table1)
>      .join(
>          Table2, Table2.y = Table1.y
>      )
>      .filter(Table1.x = xx)
>      .all()
> )
>
>     This is essentially the way I write such code.  This gives you most
> of the important advantages you seek.  You do need one extra set of
> parentheses around the whole thing, but you never have to move or alter
> those parentheses.  You can freely add new lines (i.e., add the ".all()"
> line if it wasn't there, as you suggested in a later post) without
> interfering with existing lines.  Moreover, some of the reasons you cite
> in your later post for wanting indentation enforcement become less
> important if you do things this way, because you are less likely to add
> a parenthesis in the wrong place when your parentheses are clearly set
> off like this.  (It's true, though, that it might help even more if
> Python would enforce the requirement that, when a line ends with an open
> parenthesis, its counterpart must appear at the same indentation level.)

That's a good point.  I've been waffling about where to put the closing 
parenthesis.  If flake8 had an option to force the closing parenthesis 
to be on its own line and at the same indentation level as the line with 
the opening parenthesis, I would certainly use it.

>     In my experience, the tradeoff between compactness and clarity is
> rarely a major issue, because most expressions are either simple enough
> that clarity is not at risk, or complex enough that compactness is not a
> real benefit.  If your expression is two or three lines long with no
> long nested parentheses, it will probably be clear enough even if you
> fudge a bit on the formatting.  If your expression is ten lines long,
> having to add an extra line or two (or even three) for closing
> parentheses is not such a big deal.  Basically, adding extra lines for
> parentheses is only a pain if the expression was previously "short" and
> now becomes "long", but not many expressions are very close to that
> boundary.

You may be right, but I'm not sure your experience matches mine.  I seem 
to recall changing single-line expressions into multi-line expressions 
fairly often.  I'll pay more attention in the future.

>     All that said, I do think it is worth considering some syntax to
> make this sort of thing easier.  I'm not sure the Ellipsis approach is
> the right one, but I'm interested to see where this thread goes.

Thanks for the well considered thoughts.

Shane