[Python-Dev] PEP 393 Summer of Code Project (original) (raw)

Stefan Behnel stefan_ml at behnel.de
Tue Aug 23 17🔞13 CEST 2011

Antoine Pitrou, 23.08.2011 16:08:

On Tue, 23 Aug 2011 16:02:54 +0200 Stefan Behnel wrote:

"Martin v. Löwis", 23.08.2011 15:17:

Has this been considered before? Was there a reason to decide against it?

I think we simply didn't consider it. An early version of the PEP used the lower bits for the pointer to encode the kind, in which case it even stopped being a pointer. Modules are not expected to access this pointer except through the macros, so it may not matter that much. The difference is that you could access them directly in a safe way, if it was a union. So, for an efficient character loop, replicated for performance reasons or for character range handling reasons or whatever, you could just check the string kind and then jump to the loop implementation that handles that type, without using any further macros. Macros are useful to shield the abstraction from the implementation. If you access the members directly, and the unicode object is represented differently in some future version of Python (say e.g. with tagged pointers), your code doesn't compile anymore.

Even with tagged pointers, you could just provide a macro that unpacks the pointer to the buffer for a given string kind. I don't think there's much more to be done to keep up the abstraction. I don't see a reason to prevent users from accessing the memory buffer directly, especially not by (accidental, as I understand it) obfuscation through a void*.

Stefan

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