Use posix_prefix for Apple's system Python

src/pip/_internal/locations/_sysconfig.py

@@ -15,8 +15,8 @@
 
 
 # Notes on _infer_* functions.
-# Unfortunately ``_get_default_scheme()`` is private, so there's no way to
-# ask things like "what is the '_prefix' scheme on this platform". These
+# Unfortunately ``get_default_scheme()`` didn't exist before 3.10, so there's no
+# way to ask things like "what is the '_prefix' scheme on this platform". These
 # functions try to answer that with some heuristics while accounting for ad-hoc
 # platforms not covered by CPython's default sysconfig implementation. If the
 # ad-hoc implementation does not fully implement sysconfig, we'll fall back to
@@ -27,6 +27,32 @@
 _PREFERRED_SCHEME_API = getattr(sysconfig, "get_preferred_scheme", None)
 
 
+def _should_use_osx_framework_prefix() -> bool:
+    """Check for Apple's ``osx_framework_library`` scheme.
+
+    Python distributed by Apple's Command Line Tools has this special scheme
+    that's used when:
+
+    * This is a framework build.
+    * We are installing into the system prefix.
+
+    This does not account for ``pip install --prefix`` (also means we're not
+    installing to the system prefix), which should use ``posix_prefix``, but
+    logic here means ``_infer_prefix()`` outputs ``osx_framework_library``. But
+    since ``prefix`` is not available for ``sysconfig.get_default_scheme()``,
+    which is the stdlib replacement for ``_infer_prefix()``, presumably Apple
+    wouldn't be able to magically switch between ``osx_framework_library`` and
+    ``posix_prefix``. ``_infer_prefix()`` returning ``osx_framework_library``
+    means its behavior is consistent whether we use the stdlib implementation
+    or our own, and we deal with this special case in ``get_scheme()`` instead.
+    """
+    return (
+        "osx_framework_library" in _AVAILABLE_SCHEMES
+        and not running_under_virtualenv()
+        and is_osx_framework()
+    )
+
+
 def _infer_prefix() -> str:
     """Try to find a prefix scheme for the current platform.
 
@@ -43,8 +69,7 @@ def _infer_prefix() -> str:
     """
     if _PREFERRED_SCHEME_API:
         return _PREFERRED_SCHEME_API("prefix")
-    os_framework_global = is_osx_framework() and not running_under_virtualenv()
-    if os_framework_global and "osx_framework_library" in _AVAILABLE_SCHEMES:
+    if _should_use_osx_framework_prefix():
         return "osx_framework_library"
     implementation_suffixed = f"{sys.implementation.name}_{os.name}"
     if implementation_suffixed in _AVAILABLE_SCHEMES:
@@ -130,6 +155,12 @@ def get_scheme(
     else:
         scheme_name = _infer_prefix()
 
+    # Special case: When installing into a custom prefix, use posix_prefix
+    # instead of osx_framework_library. See _should_use_osx_framework_prefix()
+    # docstring for details.
+    if prefix is not None and scheme_name == "osx_framework_library":
+        scheme_name = "posix_prefix"
+
     if home is not None:
         variables = {k: home for k in _HOME_KEYS}
     elif prefix is not None: