[Python-Dev] Re: Prospective Peephole Transformation

Skip Montanaro skip at pobox.com
Fri Feb 18 15:57:39 CET 2005

    >> 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])"....

    Fredrik> savings in what?  time or bytecode size?  constructed
    Fredrik> micro-benchmarks, or examples from real-life code?

    Fredrik> do we have any statistics on real-life "n" values?

My original suggestion wasn't based on performance issues.  It was based on
the notion of tuples-as-records and lists-as-arrays.  Raymond had originally
gone through the code and changed

    for x in [1,2,3]:


    for x in (1,2,3):

I suggested that since the standard library code is commonly used as an
example of basic Python principles (that's probably not the right word), it
should uphold that ideal tuple/list distinction.  Raymond then translated

    for x in [1,2,3]:


    for x in frozenset([1,2,3]):

I'm unclear why the list in "for x in [1,2,3]" or "if x not in [1,2,3]"
can't fairly easily be recognized as a constant and just be placed in the
constants array.  The bytecode would show n LOAD_CONST opcodes followed by
BUILD_LIST then either a COMPARE_OP (in the test case) or GET_ITER+FOR_ITER
(in the for loop case).  I think the optimizer should be able to recognize
both constructs fairly easily.

I don't know if that would provide a performance increase or not.  I was
after separation of functionality between tuples and lists.


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