[Python-Dev] Temporary Constantification (original) (raw)

Guido van Rossum guido at python.org
Mon Jun 26 01:29:41 CEST 2006

On 6/25/06, Eric Sumner <kd5bjo at gmail.com> wrote:

On 6/25/06, Guido van Rossum <guido at python.org> wrote: > Unfortunately, a mechanism that would let you register a callback for > when a particular variable or attribute used in a cached expression is > used, is pretty hard to implement without affecting the performance of > code that doesn't use it. I'm afraid this is not a very likely path > towards a solution.

I could make a strong argument that it is actually impossible to implement without affecting the performance of other code; the only issue is whether or not the impact is acceptable. I may be wrong, but I think that this particular scheme minimizes the impact: - There is a bit more data to store in every namespace - There is no change to dereferencing names; no test is required, no callback is generated - Binding to a name that currently has no binding simply requires allocating the extra memory and clearing it. - Binding to a name that is bound and does have callbacks is slow, but those are supposed to be constant in practice anyway. - Binding to a name that is already bound, but has no callbacks requires a test on a single variable against a constant. Without knowing more about the internals of Python (such as how long a check of a single variable takes relative to binding a new value to a name), I can't properly evaluate how much of a problem this would be.

Your proposal would require a change to the dict type to set a callback to be called when a particular key is modified (not a generic callback when any key is modified).

That seems pretty tricky to do with no impact, given how highly dicts are optimized.

Also, allowing attribute references is a whole new can of worms, since attributes aren't necessarily implemented as standard namespaces implemented by dictionaries. Aahz already pointed this out.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

More information about the Python-Dev mailing list