1. Overview

As package build process is controlled by distutils (superseded by setuptools), it’s important to know the current status of distutils and setuptools, why there are mutiple instances of an installation, how it’s possible that different packages uses different versions and understand what is the win-arm64 current support of them and what could be expected in short and long term.

2. Important issues

https://github.com/pypa/setuptools/issues/2372

https://www.python.org/dev/peps/pep-0632/

3. Different distutils instances

3.1. Built-in lib in Python interpreter

Historical distutils of Python, included in CPython

example path in Windows

 c:\Users\<user>\AppData\Local\Programs\Python\<Python version>\Lib\distutils\

This is going to be deprecated as PEP632 defines above.
Source: https://github.com/python/cpython/tree/main/Lib/distutils
Update
- officially only with new Python installation can be updated
- unofficially the directory can be overwritten by newer version
venv
- Not copied into venv
Usage: default
- This is invoked by Python if the SETUPTOOLS_USE_DISTUTILS environment variable is either:
  - SETUPTOOLS_USE_DISTUTILS=
  - SETUPTOOLS_USE_DISTUTILS=stdlib

3.2. Within stand-alone setuptools package

new, forked version
Source: https://github.com/pypa/setuptools/tree/main/setuptools/_distutils
Update
- as a stand-alone Python package, setuptools can be updated
- ```
pip install -U setuptools
```
- ```
python -m pip install -U setuptools
```

example path in Windows

c:\Users\<user>\AppData\Local\Programs\Python\<Python version>\Lib\site-packages\setuptools\_distutils\

venv
- It is copied into venv
- Can be updated and used regardless of global setuptools package, if venv is activated
- example path
```
<venv dir>\Lib\site-packages\setuptools\_distutils\
```
Usage: can be forced to use by SETUPTOOLS_USE_DISTUTILS environment variable
- ```
SETUPTOOLS_USE_DISTUTILS=local
```

4. WoA (win-arm64) support

Fake native compilation, no real cross-compilation

From Python point of view, we can only invoke native compilation on x64 and win-arm64 as well.
- https://github.com/pypa/setuptools/blob/main/setuptools/_distutils/command/build_ext.py#L312
- (self.plat_name == get_platform()) is set to true by our usecases
But it's cross-compilation by the C compiler (MSVC), so I tend to call it fake native compilation as we convince Python to invoke the native build flow by a cross-compiler.
- By VSCMD_ARG_TGT_ARCH env var, we're telling to Python it runs on the given platform
- As there is no native compiler for win-arm64 this works (for now) - as seen in sources below - win-arm64 platform always results x86/arm64 compiler.
  - VSCMD_ARG_TGT_ARCH=arm64 -> x86_arm64 (PLAT_TO_VCVARS) -> VC\Tools\MSVC\<version>\bin\HostX86\ARM64\cl.exe
build_ext has plat_name as cross-compiling target platform parameter, which looks hacky at first sight and I suppose that's why it’s not preferred to use
I see one long-term problem here:
- If there will be ever a native win-arm64/win-arm64 MSVC compiler, this method should be updated heavily as it can't fallback to win-arm64 = x86/win-arm64 anymore.
  - The current fake cross-compilation would fail as it would try to use the real native one.
- It’s not a blocker, but it should be known.

4.1. Built-in lib in Python interpreter

Described https://linaro.atlassian.net/wiki/spaces/WOA/pages/28593914166/setuptools-distutils#3.1.-Built-in-lib-in-Python-interpreter
As this is going to be deprecated, I won’t expect any further support.
Short summary
- On WoA machine, the cross-compile MSVC is called by default, but on x64 it can’t be enforced.

win-arm64 has been added as a host platform (in 2019)

This means on win-arm64 platform without any env var setting, it will invoke x86/arm64 MSVC compiler by default.
- Which is good on WoA

https://github.com/python/cpython/blob/v3.10.0rc1/Lib/distutils/util.py#L39

def get_host_platform():
    if os.name == 'nt':
        if 'amd64' in sys.version.lower():
            return 'win-amd64'
        if '(arm)' in sys.version.lower():
            return 'win-arm32'
        if '(arm64)' in sys.version.lower():
            return 'win-arm64'
        return sys.platform

https://github.com/python/cpython/blob/v3.10.0rc1/Lib/distutils/_msvccompiler.py#L163

    PLAT_TO_VCVARS = {
        'win32' : 'x86',
        'win-amd64' : 'x86_amd64',
        'win-arm32' : 'x86_arm',
        'win-arm64' : 'x86_arm64'
    }

win-arm64 is not added as a target platform
- it means win-arm64 as build target can’t be enforced by VSCMD_ARG_TGT_ARCH=arm64 environment variable on x64
- as this is going to be deprecated and not recommended to use for any projects it’s not a blocker, but any project that is tied to old distutils, can’t be cross-compiled from x64 to win-arm64, for example numpy!
  - see https://linaro.atlassian.net/wiki/spaces/WOA/pages/28593914166/setuptools-distutils#2.-Important-issues
- https://github.com/python/cpython/blob/v3.10.0rc1/Lib/distutils/util.py#L99
```
def get_platform():
    if os.name == 'nt':
        TARGET_TO_PLAT = {
            'x86' : 'win32',
            'x64' : 'win-amd64',
            'arm' : 'win-arm32',
        }
        return TARGET_TO_PLAT.get(os.environ.get('VSCMD_ARG_TGT_ARCH')) or get_host_platform()
    else:
        return get_host_platform()
```

Test

Test command:
    python setup.py build_ext

CPython(3.10_rc1)/distutils (old)
SETUPTOOLS_USE_DISTUTILS=
    x64
        VSCMD_ARG_TGT_ARCH=
            self.plat_name: win32
            get_platform() win32
            get_host_platform() win32
            self.cc C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\x86\cl.exe        
        VSCMD_ARG_TGT_ARCH=arm64
            self.plat_name: win32
            get_platform() win32
            get_host_platform() win32
            self.cc C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\x86\cl.exe
            
            Can't build for win-arm64!

    win-arm64
        VSCMD_ARG_TGT_ARCH=
            self.plat_name: win-arm64
            get_platform() win-arm64
            get_host_platform() win-arm64
            self.cc C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\ARM64\cl.exe

        VSCMD_ARG_TGT_ARCH=arm64
            self.plat_name: win-arm64
            get_platform() win-arm64
            get_host_platform() win-arm64
            self.cc C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\ARM64\cl.exe        
        
        Tested projects:
            Cython: successful
            numpy:  successful

4.2. Within stand-alone setuptools package

Described https://linaro.atlassian.net/wiki/spaces/WOA/pages/28593914166/setuptools-distutils#3.2.-Within-stand-alone-setuptools-package
As this deprecates the internal lib of CPython, any further support should be added and expected here.
Short summary
- On WoA machine, the cross-compile MSVC is called by default, while on x64 it can be enforced by VSCMD_ARG_TGT_ARCH=arm64 env var.

win-arm64 is supported as host platform

https://github.com/pypa/setuptools/blob/main/setuptools/_distutils/util.py#L40

def get_host_platform():
    if os.name == 'nt':
        if 'amd64' in sys.version.lower():
            return 'win-amd64'
        if '(arm)' in sys.version.lower():
            return 'win-arm32'
        if '(arm64)' in sys.version.lower():
            return 'win-arm64'
        return sys.platform

https://github.com/pypa/setuptools/blob/main/setuptools/_distutils/_msvccompiler.py#L92

    PLAT_TO_VCVARS = {
        'win32' : 'x86',
        'win-amd64' : 'x86_amd64',
        'win-arm32' : 'x86_arm',
        'win-arm64' : 'x86_arm64'
    }

win-arm64 is supported as a target platform as well

https://github.com/pypa/setuptools/blob/0c302d232dd1a1f4ec531050532a84274edac2f4/setuptools/_distutils/util.py#L100

def get_platform():
    if os.name == 'nt':
        TARGET_TO_PLAT = {
            'x86' : 'win32',
            'x64' : 'win-amd64',
            'arm' : 'win-arm32',
            'arm64': 'win-arm64',
        }
        return TARGET_TO_PLAT.get(os.environ.get('VSCMD_ARG_TGT_ARCH')) or get_host_platform()
    else:
        return get_host_platform()

Test

Test command:
    python setup.py build_ext

setuptools-57.4.0/distutils (new)
SETUPTOOLS_USE_DISTUTILS=local
    x64
        VSCMD_ARG_TGT_ARCH=
            self.plat_name: win32
            get_platform() win32
            get_host_platform() win32
            self.cc C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\x86\cl.exe
        VSCMD_ARG_TGT_ARCH=arm64
            self.plat_name: win-arm64
            get_platform() win-arm64
            get_host_platform() win32
            self.cc C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\ARM64\cl.exe

            Tested projects:
                Cython: successful
                numpy:  failed (as numpy relies on old distutils)
    win-arm64
        VSCMD_ARG_TGT_ARCH=
            self.plat_name: win-arm64
            get_platform() win-arm64
            get_host_platform() win-arm64
            self.cc C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\ARM64\cl.exe
        VSCMD_ARG_TGT_ARCH=arm64
            self.plat_name: win-arm64
            get_platform() win-arm64
            get_host_platform() win-arm64
            self.cc C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30037\bin\HostX86\ARM64\cl.exe

        Tested projects:
            Cython: successful
            numpy:  failed (as numpy relies on old distutils)

setuptools-distutils

1. Overview

2. Important issues

3. Different distutils instances

3.1. Built-in lib in Python interpreter

3.2. Within stand-alone setuptools package

4. WoA (win-arm64) support

Fake native compilation, no real cross-compilation

4.1. Built-in lib in Python interpreter

Test

4.2. Within stand-alone setuptools package

Test