[Python-Dev] Re: [Idle-dev] Forward progress with full backward compatibility

[Peter Funk]

Hi!

David Scherer on idle-dev@python.org:
[...]
> in the interpreter* is fast.  In principle, one could put THREE operators in
> the language: one with the new "float division" semantics, one that divided
> only integers, and a "backward compatibility" operator with EXACTLY the old
> semantics:
[...]
> An outline of what I did:
[...]

Yes, this really clever.  I like the ideas.

[me]:
> > 2. What should the new Interpreter do, if he sees a source file without a
> >    pragma defining the language level?  There are two possibilities:
[...]
> >    2. Assume, it is a new source file and apply language level 2 to it.
> >       This has the disadvantage, that it will break any existing code.

> I think the answer is 2.  A high-quality script for adding the pragma to
> existing files, with CLI and GUI interfaces, should be packaged with Python.
> Running it on your existing modules would be part of the installation
> process.

Okay.  But what is with the Python packages available on 
the Internet?  May be the upcoming dist-utils should handle this?
Or should the Python core distribution contain a clever installer
program, which handles this?

> Long-lived modules should always have a language level, since it makes them
> more robust against changes and also serves as documentation.  A version
> statement could be encouraged at the top of any nontrivial script, e.g:
> 
> python 1.6
[...]

global python_1_5   #implies global old_division
  or
global python_1_6   #implies global old_division
  or 
global python_1_7   #may be implies global new_division

may be we can solve another issue just discussed on python_dev with

global source_iso8859_1
  or
global source_utf_8

Cute idea... but we should keep the list of such pragmas short.

> Personally, I think that it makes more sense to talk about ways to
> gracefully migrate individual changes into the language than to put off
> every backward-incompatible change to a giant future "flag day" that will
> break all existing scripts.  Versioning of some sort should be encouraged
> starting *now*, and incorporated into 1.6 before it goes final.

Yes.

> Indeed, but Guido has spoken:
> 
> > Great ideas there, Bruce!  I hope you will post these to an
> > appropriate mailing list (perhaps idle-dev, as there's no official SIG
> > to discuss the Python 3000 transition yet, and python-dev is closed).

May be someone can invite you into 'python-dev'?  However the archives
are open to anyone and writing to the list is also open to anybody.
Only subscription is closed.  I don't know why.

Regards, Peter
P.S.: Redirected Reply-To: to David and python-dev@python.org !
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