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As package build process is controlled by distutils (superseded by setuptools), it’s important to know the current status of distutils and setuptools, why there are mutiple instances of an installation, how it’s possible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

how to handle numpy.distutils and setuptools interaction · Issue #2372 · pypa/setuptools

PEP 632 – Deprecate distutils module | peps.python.org

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: setuptools/setuptools/_distutils at main · pypa/setuptools

• 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

Limitation of the 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 it’s not a perfectly clean cross-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 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 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 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

4.2. Within stand-alone setuptools package

• Described 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