[Python-Dev] pyc files, constant folding and borderline portability issues (original) (raw)

Jared Grubb jared.grubb at gmail.com
Tue Apr 7 21:55:10 CEST 2009

On 7 Apr 2009, at 11:59, Alexandru Moșoi wrote:

Not necessarily. For example C/C++ doesn't define the order of the operations inside an expression (and AFAIK neither Python) and therefore folding 2 * 3 is OK whether b is an integer or an arbitrary object with mul operator overloaded. Moreover one would expect * to be associative and commutative (take a look at Python strings); if a * 2 * 3 returns a different result from a * 6 I will find it very surprising and probably reject such code.

That's not true. All ops in C/C++ have associativity that is fixed and
well-defined; the star op is left-associative: 23x is (23)x is 6x x23 is (x2)3, and this is NOT x6 (You can show this in C++ by
creating a class that has a side-effect in its * operator).

The star operator is not commutative in Python or C/C++ (otherwise
what would rmul do?). It's easier to see that + is not
commutative: "abc"+"def" and "def"+"abc" are definitely different!

You may be confusing the "order is undefined" for the evaluation of
parameter lists in C/C++. Example: foo(f(),g()) calls f and g in an
undefined order.

Jared

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