pyc files, constant folding and borderline portability issues
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. However you can fix the order of operations like this: a = (b * 2) * 3 or a = b * (2 * 3) or a = b * 2 a = a * 3 -- Alexandru Moșoi http://alexandru.mosoi.googlepages.com
On 7 Apr 2009, at 11:59, Alexandru Moșoi wrote:
That's not true. All ops in C/C++ have associativity that is fixed and well-defined; the star op is left-associative: 2*3*x is (2*3)*x is 6*x x*2*3 is (x*2)*3, and this is NOT x*6 (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
On 7 Apr 2009, at 11:59, Alexandru Moșoi wrote:
That's not true. All ops in C/C++ have associativity that is fixed and well-defined; the star op is left-associative: 2*3*x is (2*3)*x is 6*x x*2*3 is (x*2)*3, and this is NOT x*6 (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
participants (4)
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Alexandru Moșoi -
Fredrik Johansson -
Jared Grubb -
Terry Reedy