[Pythonmac-SIG] Need M1 Mac to Test

[Ned Deily]

On Jan 12, 2022, at 15:53, jack.jansen at cwi.nl wrote:
> Python runs just fine natively on M1 Macs. The python.org installer will install a Universal2 binary (a single binary containing both the i386 and arm architectures).
> 
> If you install through brew you get the architecture your brew has been configured for.
> 
> But there _is_ a problem with native arm Python, and that is that not all extension packages (think: from pypi) have M1 versions available yet. This is especially true for packages that depend on external libraries (which may not have been ported to M1 yet).
> 
> The way I handle this on my (M1) development machine is that I have both versions of brew installed, which is easy because arm-brew installs into /opt/homebrew by default and intel-brew installs into /usr/local.
> 
> If I open a Terminal window it has /opt/homebrew/bin before /usr/local/bin in my $PATH, and the background is white.
> 
> If I open an iTerm window it runs in Rosetta intel emulation mode, it has /usr/local/bin before /opt/homebrew/bin, and the background is a creamy old-fashioned looking off-white color.
> 
> As long as I do builds in the window of the correct color, and the correct tools I need for building have been installed in the correct directory everything works fine.The white windows will use python3 for arm, the cream windows will use python3 for intel.

That is one good solution.

For those using a universal2 (M1 arm64 and Intel-64 x86_64) build like those from the python.org installers or built from scratch with --with-universal-archs=universal2, 

./configure --enable-universalsdk --with-universal-archs=universal2 --prefix=[...] or --enable-framework=[...] ...
make -j2 && make install

note that a separate python3{x}-intel64 Intel-only executable is installed along with the dual architecture python3{x} executable. In theory it is possible to select which architecture a multi-arch executable is to run under when there is more than one option by using the "arch" command, like here to force running in Intel emulation mode under Rosetta2 on an M1 Mac:

arch -x86_64 /path/to/python3{x}

But there is a big gotcha with that: if anything running under that non-default arch Python spins off another Python in a subprocess by using the value of sys.executable to find the running interpreter binary, the "arch -x86_64" is effectively lost and the interpreter in the subprocess will run in the default architecture. This happens, for instance, when running Python's own test suite: the top-level Python process running regrtest will be running in Intel emulation but tests running in subprocesses will still be running in the default arm64 arch, possibly giving errors or silently producing misleading results. Running the tests using python3{}-intel64 avoids that problem.

Also note that, as of today, we still don't officially support cross-building of single arch Pythons on macOS, that is, building an Intel-only Python on an M1 or vice-versa. If you need such a beast, what should work is building a universal2 version on either type of Mac and then using "lipo" on the executables and libraries as needed to produce a single arch set of files. Of course, that assumes that any third-party libraries needed for the build of Python have to also have been built with both archs.

--
  Ned Deily
  nad at python.org -- []