Why should python compiled with VS2010 have different error codes than one compiled with the older compilers?  We could revisit this in a new python version, but for now, we should maintain compatibility across compilers.

the errno codes (EAGAIN etc) are provided only as a compatibility for posix apps that test "errno".  On windows, we use the WSA return values from the api functions and WsaGetLastError().

The reason this patch is required is that now, the errno constants get pulled in with the WSA ones, and therefore the conditional compilation in errnomodule.c gets mangled.  So, the proposed patch is not a change, it is merely reinforcing the previous practice of prefering the native error codes over the 'errno' emulation.  Of course, this should be clarified in a comment, as you point out.