With 3.9.1 source code, I am trying to make univerisal2 build of python.framework from the source code. The problem I am seeing is the main python binary is universal, but all the .so files in lib-dynload (Python.framework/Versions/3.9/lib/python3.9/lib-dynload/) are only x86_64.

The Mac is MacOS 10.15, with Xcode 12.2 installed. Xcode12.2 has SDKs to support universal build for both arm64 and x86_64 architecture. I have no problems making other universal applications and frameworks on the machine. 

Following the documentation. I ran the following build command:
./configure --enable-universalsdk --enable-framework=./tmp --with-universal-archs=universal2 --without-ensurepip
make
make install

The resulting python was universal but the .so files are not.

Currently macOS 11 is needed to build a universal2 framework. That because we haven't spent time yet on detecting if the current compiler can compile for arm64.

An easy workaround: Patch _osx_support.py, in particular the function "_supports_arm64_builds". This checks that the macOS version is 11.0 or later.



A PR that detects if Xcode 12 or the corresponding command line tools are used  would be welcome :-)

I've added a (draft) PR for this. The PR is a this time not tested on a 10.15 system.

Another change that might be needed: ensure that sysconfig.get_platform() does not return "universal2" as the binary architecture when the compiler does not support arm64.

> Another change that might be needed: ensure that sysconfig.get_platform() 
> does not return "universal2" as the binary architecture when the compiler 
> does not support arm64.

Hmmm... _osx_support.get_platform_osx() contains the following comment:

        # Use the original CFLAGS value, if available, so that we
        # return the same machine type for the platform string.
        # Otherwise, distutils may consider this a cross-compiling
        # case and disallow installs.

That indicates that fixing the value of sysconfig.get_platform() when using a "universal2" python on a system that doesn't support arm64 might break distils. 

I haven't done further research on this. In general I'd prefer to create the correct platform tag, but not if that results in not being able to install extensions.