The legacy Unicode C API was deprecated in 3.3. Its support consumes resources: more memory usage by Unicode objects, additional code for handling Unicode objects created with the legacy C API. Currently every Unicode object has a cache for the wchar_t representation.

The proposed PR adds two compile time options: HAVE_UNICODE_WCHAR_CACHE and USE_UNICODE_WCHAR_CACHE. Both are set to 1 by default.

If USE_UNICODE_WCHAR_CACHE is set to 0, CPython will not use the wchar_t cache internally. The new wchar_t based C API will be used instead of the Py_UNICODE based C API. This can add small performance penalty for creating a temporary buffer for the wchar_t representation. On other hand, this will decrease the long-term memory usage. This build is binary compatible with the standard build and third-party extensions can use the legacy Unicode C API.

If HAVE_UNICODE_WCHAR_CACHE is set to 0, the wchar_t cache will be completely removed. The legacy Unicode C API will be not available, and functions that need it (e.g. PyArg_ParseTuple() with the "u" format unit) will always fail. This build is binary incompatible with the standard build if you use the legacy or non-stable Unicode C API.

I hope that these options will help third-party projects to prepare for removing the legacy Unicode C API in future.