[Python-Dev] Re: Update PEP 292

[Barry Warsaw]

On Wed, 2004-08-11 at 08:06, Fernando Perez wrote:

> If I understood things correctly, this proposal will not allow the evaluation of
> any kind of expression, including attributes.  With the existing interpolation
> system, the following happens:
> 
> In [2]: print 'sys.platform is: %(sys.platform)s' % locals()
> ---------------------------------------------------------------------------
> KeyError                                  Traceback (most recent call last)
> 
> ?
> 
> KeyError: sys.platform

You don't mean the 'existing' interpolation system, because this
obviously doesn't work in Python today, although with a fancy enough
implementation of 'locals()', it could.  That actually doesn't change
much with PEP 292.  I specifically designed it so that you could
subclass from template, so as to accept the extended placeholder syntax
(i.e. dots), and then implement a mapping-like class to do the lookups
in any namespace you want.  In fact, my earlier implementation contained
such a class, but it was deemed too complex to win the name 'simpler' so
it was left out of the library.  It's a trivial amount of code and my
own personal use case has a need for such a feature.

-Barry

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