[Python-ideas] Reporting unmatched parentheses in SyntaxError messages?

[Steven D'Aprano]

On Wed, Jul 08, 2015 at 11:03:42AM +0200, Todd wrote:
> On Jul 8, 2015 10:02 AM, "Stephen J. Turnbull" <stephen at xemacs.org> wrote:
> >
> > Nick Coghlan writes:
> >
> >  > While I have no idea how we could implement it, I'm wondering if that
> >  > might be clearer if the error message instead looked more like this:
> >  >
> >  >     File "/home/me/myfile.py", line 11
> >  >         data = func()
> >  >         ^
> >  >     SyntaxError: invalid syntax (Unmatched '(' on line 10)
> >
> > I think I would prefer "Expected ')'".  I think that typos like
> >
> >     a = ((1, "one"),
> >          (2, "two)",
> >          (3, "three"))
> >     data = func()
> >
> > are likely to be fairly common (I make them often enough!), but I
> > don't see how you're going to get the parser to identify the line
> > containing "couple #2" as the source of the error (without a *really*
> > dubious heuristic).
> 
> True, but we can definitely say it occurs on or after the first line in
> your example. So could we do something like:
> 
>      File "/home/me/myfile.py", line 11
>          data = func()
>          ^
>    SyntaxError: Unmatched '(' starting somewhere after line 7
> 
> That would at least allow you to narrow down where to look for the problem.

Even if you can't report a line number, you could report "on this or a 
previous line".

The way I see it, if the parser knows enough to point the ^ before the 
first token on the line, it can report that there is a missing ) on a 
previous line, otherwise it may have to hedge.

SyntaxError: Unmatched '(' before this line

SyntaxError: Unmatched '(' on this or a previous line

I believe that this would be a big help to beginners and casual users 
such as sys admins. Experienced programmers have learned the hard way 
that a SyntaxError may mean an unmatched bracket of some kind, but I 
think it would help even experienced coders to be explicit about the 
error.

-- 
Steve