[Python-ideas] Extending expressions using ellipsis

[Chris Kaynor]

Guido's time machine strikes again, though using a slash (\) rather than
elipse:

>>> '.'\
... .join(
...     (
...             '1',
...             '2',
...     )
... )
'1.2'

This is from Python 2.7.10 (what I have on the machine I am currently on),
though I'm fairly sure it has worked for quite a bit longer than that.

Chris

On Wed, Aug 31, 2016 at 2:46 PM, Shane Hathaway <shane at hathawaymix.org>
wrote:

> Hi,
>
> I write a lot of SQLAlchemy code that looks more or less like this:
>
> rows = (
>     dbsession.query(Table1)
>     .join(
>         Table2, Table2.y = Table1.y)
>     .filter(Table1.x = xx)
>     .all())
>
> The expressions get very long and nearly always need to be spread to
> multiple lines. I've tried various styles and have chosen the style above
> as the most tasteful available.
>
> Pros of the existing syntax:
>
> - It's possible to indent clearly and consistently.
> - Nested indentation works out OK.
> - It's flexible; I can combine lines or separate them for emphasis.
>
> Cons:
>
> - Extra parentheses are required.
> - The indentation is not enforced by the parser, so I have unnecessary
> freedom that could let various mistakes slip through.
> - The closing parenthesis has to move every time I append to or reorder
> the expression, leading to diff noise in version control. (Alternatively, I
> could put the closing parenthesis on its own line, but that consumes
> precious vertical reading space.)
>
> I'd like to suggest a small change to the Python parser that would make
> long expressions read better:
>
> rows = dbsession.query(Table1) ...
>     .join(
>         Table2, Table2.y = Table1.y)
>     .filter(Table1.x = xx)
>     .all()
>
> The idea is to use an ellipsis at the end of a line to spread an
> expression over multiple indented lines, terminated by a return to an
> earlier indentation level.  You can still indent more deeply as needed, as
> shown above by the join() method call.
>
> This syntax has all the pros of the existing syntax and resolves all the
> cons:
>
> - No extra parentheses are required.
> - The indentation is enforced, so my mistakes are more likely to be caught
> early.
> - Without a closing parenthesis, there is no diff noise when I append to
> or reorder an expression.
>
> I've thought about using a colon instead of an ellipsis, but in Python, a
> colon starts a list of statements; that's not my goal. Instead, I'm looking
> for ways to use parser-enforced indentation to avoid mistakes and help my
> code read better without changing any semantics.
>
> Feedback is welcome!
>
> Shane
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