Add a templated cross-platform sitecustomize.py to non-macOS support packages.

Author: freakboy3742

Makefile

@@ -872,8 +872,7 @@ $$(PYTHON_STDLIB-$(os)): \
 	$$(foreach sdk,$$(SDKS-$(os)),cp $$(PYTHON_FATSTDLIB-$$(sdk))/lib-dynload/* $$(PYTHON_STDLIB-$(os))/lib-dynload; )
 
 dist/Python-$(PYTHON_VER)-$(os)-support.$(BUILD_NUMBER).tar.gz: $$(PYTHON_XCFRAMEWORK-$(os)) $$(PYTHON_STDLIB-$(os))
-	@echo ">>> Create final distribution artefact for $(os)"
-	mkdir -p dist
+	@echo ">>> Create VERSIONS file for $(os)"
 	echo "Python version: $(PYTHON_VERSION) " > support/$(os)/VERSIONS
 	echo "Build: $(BUILD_NUMBER)" >> support/$(os)/VERSIONS
 	echo "Min $(os) version: $$(VERSION_MIN-$(os))" >> support/$(os)/VERSIONS
@@ -887,6 +886,17 @@ endif
 	echo "OpenSSL: $(OPENSSL_VERSION)" >> support/$(os)/VERSIONS
 	echo "XZ: $(XZ_VERSION)" >> support/$(os)/VERSIONS
 
+ifneq ($(os),macOS)
+	@echo ">>> Create cross-platform site sitecustomize.py for $(os)"
+	mkdir -p support/$(os)/platform-site
+	cat $(PROJECT_DIR)/patch/Python/sitecustomize.py \
+		| sed -e "s/{{os}}/$(os)/g" \
+		| sed -e "s/{{tag}}/$$(shell echo $(os) | tr '[:upper:]' '[:lower:]')_$$(shell echo $$(VERSION_MIN-$(os)) | sed "s/\./_/g")/g" \
+		> support/$(os)/platform-site/sitecustomize.py
+endif
+
+	@echo ">>> Create final distribution artefact for $(os)"
+	mkdir -p dist
 	# Build a "full" tarball with all content for test purposes
 	tar zcvf dist/Python-$(PYTHON_VER)-$(os)-support.test-$(BUILD_NUMBER).tar.gz -X patch/Python/test.exclude -C support/$(os) `ls -A support/$(os)`
 	# Build a distributable tarball

README.rst

@@ -90,6 +90,17 @@ Each support package contains:
   one for physical devices, and one for the simulator. The simulator binaries
   are "fat", containing code for both x86_64 and arm64.
 
+Non-macOS platforms also contain a ``platform-site`` folder. This contains a
+site customization script that can be used to make your local Python install
+look like it is an on-device install. This is needed because when you run
+``pip`` you'll be on a macOS machine; if ``pip`` tries to install a binary
+package, it will install a macOS binary wheel (which won't work on
+iOS/tvOS/watchOS). However, if you add the ``platform-site`` folder to your
+``PYTHONPATH`` when invoking pip, the site customization will make your Python
+install return ``platform`` and ``sysconfig`` responses consistent with
+on-device behavior, which will cause ``pip`` to install platform-appropriate
+packages.
+
 To add a support package to your own Xcode project:
 
 1. Drag ``Python.xcframework`` and ``python-stdlib`` into your Xcode project

patch/Python/sitecustomize.py

@@ -0,0 +1,31 @@
+# A site customization that can be used to trick pip into installing
+# packages cross-platform. If the folder containing this file is on
+# your PYTHONPATH when you invoke pip, pip will behave as if it were
+# running on {{os}}.
+import os
+import platform
+import sys
+import sysconfig
+
+# Make platform.system() return "{{os}}"
+def custom_system():
+    return "{{os}}"
+
+platform.system = custom_system
+
+# Make sysconfig.get_platform() return "{{tag}}"
+def custom_get_platform():
+    return "{{tag}}"
+
+sysconfig.get_platform = custom_get_platform
+
+# Call the next sitecustomize script if there is one
+# (https://nedbatchelder.com/blog/201001/running_code_at_python_startup.html).
+del sys.modules["sitecustomize"]
+this_dir = os.path.dirname(__file__)
+path_index = sys.path.index(this_dir)
+del sys.path[path_index]
+try:
+    import sitecustomize  # noqa: F401
+finally:
+    sys.path.insert(path_index, this_dir)