gh-129545 Improve Syntax Error Message for Default Parameter Order

[sharktide]

Fixes and closes #129545

Overview:

The SyntaxError: parameter without a default follows parameter with a default error message has been modified to SyntaxError: positional parameter without a default follows parameter with a default to clarify error messages.

As mentioned in #129545

@smheidrich

Trying to define a function in which a positional parameter follows a parameter with a default value, we get a SyntaxError as expected:

>>> def f(a=1, b):
 File "<python-input-0>", line 1
 def f(a=1, b):
            ^
SyntaxError: parameter without a default follows parameter with a default

But the message that comes with it is not as accurate as it could be, as we can easily construct a valid function signature in which a parameter without a default follows a parameter with a default so long as both are keyword-only parameters:

>>> def f(*, a=1, b):
...     pass
...     
>>> 

(I didn't use a=1, *, b for this counterexample because then one could argue that, if "follows" is interpreted as "follows immediately", we did resolve the situation described in the error message, making it formally accurate, albeit confusing.)

As also mentioned by @smheidrich:

It's not just a matter of accuracy for accuracy's sake, however: This change would also hint to people who don't know or have forgotten about keyword-only parameters that there is another possibility of resolving the issue than the ones implied by the current message.

The new error message, should be accurate for most cases and should not be confusing. However, if there are cases where the original error message might be more appropriate ex when dealing with keyword-only parameters or other specific syntax rules but the new change would cause minimal confusion compared to the problem that this pr intends to solve.

• Issue: SyntaxError: parameter without a default follows parameter with a default is inaccurate #129545

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[sharktide]

note: will add news entry tmw

[lysnikolaou]

Thanks for the PR @sharktide! I'm good with this change. I've left one comment about a formatting issue.

[lysnikolaou]

Please remove the additional trailing spaces here. They shouldn't be needed.

[sharktide]

ok

[sharktide]

@lysnikolaou Done ready for re-review/merge

[sharktide]

@lysnikolaou Does a small change need a news entry? I didn't know if to put one or not. Since you commented it looks good other than the formatting, I am assuming not, but if so, could you please apply the "skip news" label?
the bedevere-bot keeps notifying me of a missing news entry

[lysnikolaou]

@sharktide A news entry would be best here, since there's a user-facing change, so please add one.

[sharktide]

got it

[sharktide]

I'll add it under the topic c-api

[sharktide]

@lysnikolaou Added the news entry, everything done

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[bedevere-app]

Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[lysnikolaou]

One editorial comment on the news blurb. Also, a better category for it would be Core and Builtins. Maybe move it there?

[sharktide]

One editorial comment on the news blurb. Also, a better category for it would be Core and Builtins. Maybe move it there?

ok

[sharktide]

@lysnikolaou I moved the news entry to Core and Builtins as well as took your change suggestion