[Python-Dev] Optimization of Python ASTs: How should we deal with constant values? (original) (raw)

Thomas Lee tom at vector-seven.com
Fri May 2 16:21:33 CEST 2008

Martin v. Löwis wrote:

This leaves us with a few options:

5. Reuse/Abuse Num(object) for arbitrary constants. AFAICT, this should work out of the box. Eek. It does seem like Num would work out of the box, but would this be a good idea?

What about replacing Num with Const? Might make optimizations specifically for numeric values slightly hairier, and semantically I think they might be different enough to warrant separate AST nodes despite the similarity in implementation at the compiler level.

FWIW, I read Num as "numeric literal" and Const as "arbitrary constant", but that's probably only because I've seen the immediate need for constants with non-Num values in the process of writing the AST optimizer.

1. A new AST expr node for constant values for types other than Str/Num

I imagine this to be something like Const(PyObject* v), which is effectively translated to a "LOADCONST v" by the compiler. This trades the purity of the AST for a little practicality. A "Const" node has no real source representation, it would exist solely for the purpose of injecting PyObject constants into the AST. I think this is the way to go. It doesn't violate purity: it is an abstract syntax, meaning that there doesn't need to be a 1:1 relationship to source syntax. However, it is still possible to reproduce source code from this Const node. I'm leaning toward this, too. It's dirt simple and quite clean to implement. I also don't worry about Jython conflicts. The grammar has a version number precisely so that you can refer to a specific version if you need to. Any Jython folk care to weigh in on this? If there are no major objections I think I'm going to forge ahead with an independant Const() node.

Cheers, T

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