[Python-Dev] Prospective Peephole Transformation

[M.-A. Lemburg]

Raymond Hettinger wrote:
> Based on some ideas from Skip, I had tried transforming the likes of "x
> in (1,2,3)" into "x in frozenset([1,2,3])".  When applicable, it
> substantially simplified the generated code and converted the O(n)
> lookup into an O(1) step.  There were substantial savings even if the
> set contained only a single entry.  When disassembled, the bytecode is
> not only much shorter, it is also much more readable (corresponding
> almost directly to the original source).
> 
> The problem with the transformation was that it didn't handle the case
> where x was non-hashable and it would raise a TypeError instead of
> returning False as it should.  That situation arose once in the email
> module's test suite.
> 
> To get it to work, I would have to introduce a frozenset subtype:
> 
>     class Searchset(frozenset):
>         def __contains__(self, element):
>             try:
>                 return frozenset.__contains__(self, element)
>             except TypeError:
>                 return False
> 
> Then, the transformation would be "x in Searchset([1, 2, 3])".  Since
> the new Searchset object goes in the constant table, marshal would have
> to be taught how to save and restore the object.
> 
> This is a more complicated than the original frozenset version of the
> patch, so I would like to get feedback on whether you guys think it is
> worth it.

Wouldn't it help a lot more if the compiler would detect that
(1,2,3) is immutable and convert it into a constant at
compile time ?!

The next step would then be to have Python roll out these
loops (in -O mode).

-- 
Marc-Andre Lemburg
eGenix.com

Professional Python Services directly from the Source  (#1, Feb 18 2005)
 >>> Python/Zope Consulting and Support ...        http://www.egenix.com/
 >>> mxODBC.Zope.Database.Adapter ...             http://zope.egenix.com/
 >>> mxODBC, mxDateTime, mxTextTools ...        http://python.egenix.com/
________________________________________________________________________

::: Try mxODBC.Zope.DA for Windows,Linux,Solaris,FreeBSD for free ! ::::