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

[Todd]

On Wed, Jul 8, 2015 at 1:33 PM, Steven D'Aprano <steve at pearwood.info> wrote:

> 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.
>
>
I think it should always be possible to report a range of line numbers
within which the problem must occur.  The start would be the outermost
unclosed parenthesis.  The end would the first statement that cannot be in
a parentheses or the end of the file, whichever comes first (the latter is
currently listed as the location of the exception).  Although we can't say
exactly where the problem occurs in this range, I think we can say that it
must be somewhere in this range.

So it should be possible to do something like this (I don't like the error
message, this is just an example):

      File "/home/me/myfile.py", line 8:12
          a = ((1, "one"),
                ^
          ...
          data = func()
          ^
    SyntaxError: Unmatched '(' in line range

However, I don't know if this exception message structure could be a
problem.  Hence my original proposal, which would keep a simpler exception
message.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-ideas/attachments/20150708/975e5bc4/attachment.html>