[Python-Dev] PEP 384: Defining a Stable ABI (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Tue May 26 20:31:16 CEST 2009

The structure of type objects is not available to applications; declaration of "static" type objects is not possible anymore (for applications using this ABI). Hmm, that's going to create big problems for extensions that want to expose a C-API for their types: Type checks are normally done by pointer comparison using those static type objects. They would just have to expose "MyExtensionPrefixMyTypeCheck" and "MyExtensionPrefixMyTypeCheckExact" functions the same way that types in the C API do. Hmm, that's a function call per type check and will slow things down a lot, esp. when working with APIs that deal a lot with these objects.

See my other response. You can continue to provide _Check macros; knowledge of the structure of types is not necessary to perform such checks.

The typical way to implement these type checks is via a simple pointer comparison (falling back to a function for sub-types). That's cheap and fast.

And will continue to be available to ABI-compliant extensions.

Including PyINCREF()/PyDECREF() ? I believe so - MvL deliberately left the fields that the ref counting relies on as part of the ABI. Hmm, another slow-down.

??? Why is "no change" a slow-down?

This is not much of an issue if the C runtime DLL doesn't change between releases, but it becomes a problem when they do e.g. due to an upgrade to a new MSVC++ compiler version or in case the extension was downloaded pre-compiled from pypi or some other site.

What problem specifically may occur?

Regards, Martin

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