Add support for Homebrew on Python 3.9 instead of relying on distutil…

…s.cfg as found in the stdlib. Fixes #152.

distutils/command/_framework_compat.py

@@ -0,0 +1,52 @@
+"""
+Backward compatibility for homebrew builds on macOS.
+"""
+
+
+import sys
+import os
+import functools
+import subprocess
+
+
+@functools.lru_cache()
+def enabled():
+    """
+    Only enabled for Python 3.9 framework builds except ensurepip and venv.
+    """
+    PY39 = (3, 9) < sys.version_info < (3, 10)
+    framework = sys.platform == 'darwin' and sys._framework
+    venv = sys.prefix != sys.base_prefix
+    ensurepip = os.environ.get("ENSUREPIP_OPTIONS")
+    return PY39 and framework and not venv and not ensurepip
+
+
+schemes = dict(
+    osx_framework_library=dict(
+        stdlib='{installed_base}/{platlibdir}/python{py_version_short}',
+        platstdlib='{platbase}/{platlibdir}/python{py_version_short}',
+        purelib='{homebrew_prefix}/lib/python{py_version_short}/site-packages',
+        platlib='{homebrew_prefix}/{platlibdir}/python{py_version_short}/site-packages',
+        include='{installed_base}/include/python{py_version_short}{abiflags}',
+        platinclude='{installed_platbase}/include/python{py_version_short}{abiflags}',
+        scripts='{homebrew_prefix}/bin',
+        data='{homebrew_prefix}',
+    )
+)
+
+
+@functools.lru_cache()
+def vars():
+    if not enabled():
+        return {}
+    homebrew_prefix = subprocess.check_output(['brew', '--prefix'], text=True).strip()
+    return locals()
+
+
+def scheme(name):
+    """
+    Override the selected scheme for posix_prefix.
+    """
+    if not enabled() or not name.endswith('_prefix'):
+        return name
+    return 'osx_framework_library'

distutils/command/install.py

@@ -17,6 +17,7 @@
 from distutils.util import convert_path, subst_vars, change_root
 from distutils.util import get_platform
 from distutils.errors import DistutilsOptionError
+from . import _framework_compat as fw
 from .. import _collections
 
 from site import USER_BASE
@@ -82,6 +83,10 @@
         'data': '{userbase}',
     }
 
+
+INSTALL_SCHEMES.update(fw.schemes)
+
+
 # The keys to an installation scheme; if any new types of files are to be
 # installed, be sure to add an entry to every installation scheme above,
 # and to SCHEME_KEYS here.
@@ -136,7 +141,7 @@ def _resolve_scheme(name):
     try:
         resolved = sysconfig.get_preferred_scheme(key)
     except Exception:
-        resolved = _pypy_hack(name)
+        resolved = fw.scheme(_pypy_hack(name))
     return resolved
 
 
@@ -426,7 +431,7 @@ def finalize_options(self):
             local_vars['usersite'] = self.install_usersite
 
         self.config_vars = _collections.DictStack(
-            [compat_vars, sysconfig.get_config_vars(), local_vars]
+            [fw.vars(), compat_vars, sysconfig.get_config_vars(), local_vars]
         )
 
         self.expand_basedirs()