[Python-Dev] PEP 264 (was RE: [Python-checkins] CVS: python/nondist/peps pep-0264.txt,NONE,1.1 pep-0000.txt,1.113,1.114)

[Michael Hudson]

Instant response, without that tedious-prone-to-problem-spotting
thinking: nice idea!

"Tim Peters" <tim.one@home.com> writes:

> [Guido, checking in
>     Michael Hudson's "Future statements in simulated shells".
>  PEP
> ]
> > ...
> >     Paul Prescod has reasonably complained about the choice of a
> >     bitfield as the fourth argument to compile(), claiming it is
> >     obscure and unpythonic.
> 
> I've got a happy idea!  __future__.py is a real std module, and e.g.
> 
>     __future__.nested_scopes
>     __future__.generators
>     __future__.division
> 
> are honest-to-gosh Python objects, of class __future__._Feature.  That's the
> perfect place to store feature-specific magic flag bits.  So, e.g., I'm
> picturing
> 
>     compile(s1, s2, s3, __future__.generators.getflag())

Or just 

    __future__.generators.compile_flag

presumably?

> and
> 
>     flags = 0
>     for fname in list_of_feature_names:
>         flags |= getattr(__future__, fname).getflag()
>     compile(s1, s2, s3, flags)
> 
> etc.  For that matter, list-of-feature-strings -> flag_bits could be
> added as a utility function to __future__.py.  It's Python --
> there's nothing you can't do with this in 5 seconds <wink>.

"list_of_feature_names" might as well be spelt "__future__.__all__",
I'd have thought.

There's still the icky issue of PyEval_MergeCompilerFlags to consider.
The fact that each future statement has two bits assigned to in - the
PyCF_* ones in pythonrun.h and the CO_* ones in compile.h that allow
__builtin__.compile to inherit the behaviour of surrounding code -
seems nasty.  We could just mandate that 

CO_FOO == PyCF_FOO << 12

Or add another flag to code objects.  Or do something else.

The possibility of fiddling with __future__.py had completely escaped
my mind - so thanks for pointing me at that!

Cheers,
M.

-- 
  If you don't use emacs, you're a pathetic, mewling, masochistic
  weakling and I can't be bothered to convert you.    -- Ron Echeverri