6. Using Python on iOS
**********************

Authors:
   Russell Keith-Magee (2024-03)

Python on iOS is unlike Python on desktop platforms. On a desktop
platform, Python is generally installed as a system resource that can
be used by any user of that computer. Users then interact with Python
by running a **python** executable and entering commands at an
interactive prompt, or by running a Python script.

On iOS, there is no concept of installing as a system resource. The
only unit of software distribution is an "app". There is also no
console where you could run a **python** executable, or interact with
a Python REPL.

As a result, the only way you can use Python on iOS is in embedded
mode - that is, by writing a native iOS application, and embedding a
Python interpreter using "libPython", and invoking Python code using
the Python embedding API. The full Python interpreter, the standard
library, and all your Python code is then packaged as a standalone
bundle that can be distributed via the iOS App Store.

If you're looking to experiment for the first time with writing an iOS
app in Python, projects such as BeeWare and Kivy will provide a much
more approachable user experience. These projects manage the
complexities associated with getting an iOS project running, so you
only need to deal with the Python code itself.


6.1. Python at runtime on iOS
=============================


6.1.1. Platform identification
------------------------------

When executing on iOS, "sys.platform" will report as "ios". This value
will be returned on an iPhone or iPad, regardless of whether the app
is running on the simulator or a physical device.

Information about the specific runtime environment, including the iOS
version, device model, and whether the device is a simulator, can be
obtained using "platform.ios_ver()". "platform.system()" will report
"iOS" or "iPadOS", depending on the device.

"os.uname()" reports kernel-level details; it will report a name of
"Darwin".


6.1.2. Standard library availability
------------------------------------

The Python standard library has some notable omissions and
restrictions on iOS. See the API availability guide for iOS for
details.


6.1.3. Binary extension modules
-------------------------------

One notable difference about iOS as a platform is that App Store
distribution imposes hard requirements on the packaging of an
application. One of these requirements governs how binary extension
modules are distributed.

The iOS App Store requires that *all* binary modules in an iOS app
must be dynamic libraries, contained in a framework with appropriate
metadata, stored in the "Frameworks" folder of the packaged app. There
can be only a single binary per framework, and there can be no
executable binary material outside the "Frameworks" folder.

This conflicts with the usual Python approach for distributing
binaries, which allows a binary extension module to be loaded from any
location on "sys.path". To ensure compliance with App Store policies,
an iOS project must post-process any Python packages, converting ".so"
binary modules into individual standalone frameworks with appropriate
metadata and signing. For details on how to perform this post-
processing, see the guide for adding Python to your project.

To help Python discover binaries in their new location, the original
".so" file on "sys.path" is replaced with a ".fwork" file. This file
is a text file containing the location of the framework binary,
relative to the app bundle. To allow the framework to resolve back to
the original location, the framework must contain a ".origin" file
that contains the location of the ".fwork" file, relative to the app
bundle.

For example, consider the case of an import "from foo.bar import
_whiz", where "_whiz" is implemented with the binary module
"sources/foo/bar/_whiz.abi3.so", with "sources" being the location
registered on "sys.path", relative to the application bundle. This
module *must* be distributed as
"Frameworks/foo.bar._whiz.framework/foo.bar._whiz" (creating the
framework name from the full import path of the module), with an
"Info.plist" file in the ".framework" directory identifying the binary
as a framework. The "foo.bar._whiz" module would be represented in the
original location with a "sources/foo/bar/_whiz.abi3.fwork" marker
file, containing the path "Frameworks/foo.bar._whiz/foo.bar._whiz".
The framework would also contain
"Frameworks/foo.bar._whiz.framework/foo.bar._whiz.origin", containing
the path to the ".fwork" file.

When running on iOS, the Python interpreter will install an
"AppleFrameworkLoader" that is able to read and import ".fwork" files.
Once imported, the "__file__" attribute of the binary module will
report as the location of the ".fwork" file. However, the "ModuleSpec"
for the loaded module will report the "origin" as the location of the
binary in the framework folder.


6.1.4. Compiler stub binaries
-----------------------------

Xcode doesn't expose explicit compilers for iOS; instead, it uses an
"xcrun" script that resolves to a full compiler path (e.g., "xcrun
--sdk iphoneos clang" to get the "clang" for an iPhone device).
However, using this script poses two problems:

* The output of "xcrun" includes paths that are machine specific,
  resulting in a sysconfig module that cannot be shared between users;
  and

* It results in "CC"/"CPP"/"LD"/"AR" definitions that include spaces.
  There is a lot of C ecosystem tooling that assumes that you can
  split a command line at the first space to get the path to the
  compiler executable; this isn't the case when using "xcrun".

To avoid these problems, Python provided stubs for these tools. These
stubs are shell script wrappers around the underingly "xcrun" tools,
distributed in a "bin" folder distributed alongside the compiled iOS
framework. These scripts are relocatable, and will always resolve to
the appropriate local system paths. By including these scripts in the
bin folder that accompanies a framework, the contents of the
"sysconfig" module becomes useful for end-users to compile their own
modules. When compiling third-party Python modules for iOS, you should
ensure these stub binaries are on your path.


6.2. Installing Python on iOS
=============================


6.2.1. Tools for building iOS apps
----------------------------------

Building for iOS requires the use of Apple's Xcode tooling. It is
strongly recommended that you use the most recent stable release of
Xcode. This will require the use of the most (or second-most) recently
released macOS version, as Apple does not maintain Xcode for older
macOS versions. The Xcode Command Line Tools are not sufficient for
iOS development; you need a *full* Xcode install.

If you want to run your code on the iOS simulator, you'll also need to
install an iOS Simulator Platform. You should be prompted to select an
iOS Simulator Platform when you first run Xcode. Alternatively, you
can add an iOS Simulator Platform by selecting from the Platforms tab
of the Xcode Settings panel.


6.2.2. Adding Python to an iOS project
--------------------------------------

Python can be added to any iOS project, using either Swift or
Objective C. The following examples will use Objective C; if you are
using Swift, you may find a library like PythonKit to be helpful.

To add Python to an iOS Xcode project:

1. Build or obtain a Python "XCFramework". See the instructions in
   iOS/README.rst (in the CPython source distribution) for details on
   how to build a Python "XCFramework". At a minimum, you will need a
   build that supports "arm64-apple-ios", plus one of either "arm64
   -apple-ios-simulator" or "x86_64-apple-ios-simulator".

2. Drag the "XCframework" into your iOS project. In the following
   instructions, we'll assume you've dropped the "XCframework" into
   the root of your project; however, you can use any other location
   that you want by adjusting paths as needed.

3. Drag the "iOS/Resources/dylib-Info-template.plist" file into your
   project, and ensure it is associated with the app target.

4. Add your application code as a folder in your Xcode project. In the
   following instructions, we'll assume that your user code is in a
   folder named "app" in the root of your project; you can use any
   other location by adjusting paths as needed. Ensure that this
   folder is associated with your app target.

5. Select the app target by selecting the root node of your Xcode
   project, then the target name in the sidebar that appears.

6. In the "General" settings, under "Frameworks, Libraries and
   Embedded Content", add "Python.xcframework", with "Embed & Sign"
   selected.

7. In the "Build Settings" tab, modify the following:

   * Build Options

     * User Script Sandboxing: No

     * Enable Testability: Yes

   * Search Paths

     * Framework Search Paths: "$(PROJECT_DIR)"

     * Header Search Paths:
       ""$(BUILT_PRODUCTS_DIR)/Python.framework/Headers""

   * Apple Clang - Warnings - All languages

     * Quoted Include In Framework Header: No

8. Add a build step that copies the Python standard library into your
   app. In the "Build Phases" tab, add a new "Run Script" build step
   *before* the "Embed Frameworks" step, but *after* the "Copy Bundle
   Resources" step. Name the step "Install Target Specific Python
   Standard Library", disable the "Based on dependency analysis"
   checkbox, and set the script content to:

      set -e

      mkdir -p "$CODESIGNING_FOLDER_PATH/python/lib"
      if [ "$EFFECTIVE_PLATFORM_NAME" = "-iphonesimulator" ]; then
          echo "Installing Python modules for iOS Simulator"
          rsync -au --delete "$PROJECT_DIR/Python.xcframework/ios-arm64_x86_64-simulator/lib/" "$CODESIGNING_FOLDER_PATH/python/lib/"
      else
          echo "Installing Python modules for iOS Device"
          rsync -au --delete "$PROJECT_DIR/Python.xcframework/ios-arm64/lib/" "$CODESIGNING_FOLDER_PATH/python/lib/"
      fi

   Note that the name of the simulator "slice" in the XCframework may
   be different, depending the CPU architectures your "XCFramework"
   supports.

9. Add a second build step that processes the binary extension modules
   in the standard library into "Framework" format. Add a "Run Script"
   build step *directly after* the one you added in step 8, named
   "Prepare Python Binary Modules". It should also have "Based on
   dependency analysis" unchecked, with the following script content:

      set -e

      install_dylib () {
          INSTALL_BASE=$1
          FULL_EXT=$2

          # The name of the extension file
          EXT=$(basename "$FULL_EXT")
          # The location of the extension file, relative to the bundle
          RELATIVE_EXT=${FULL_EXT#$CODESIGNING_FOLDER_PATH/}
          # The path to the extension file, relative to the install base
          PYTHON_EXT=${RELATIVE_EXT/$INSTALL_BASE/}
          # The full dotted name of the extension module, constructed from the file path.
          FULL_MODULE_NAME=$(echo $PYTHON_EXT | cut -d "." -f 1 | tr "/" ".");
          # A bundle identifier; not actually used, but required by Xcode framework packaging
          FRAMEWORK_BUNDLE_ID=$(echo $PRODUCT_BUNDLE_IDENTIFIER.$FULL_MODULE_NAME | tr "_" "-")
          # The name of the framework folder.
          FRAMEWORK_FOLDER="Frameworks/$FULL_MODULE_NAME.framework"

          # If the framework folder doesn't exist, create it.
          if [ ! -d "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER" ]; then
              echo "Creating framework for $RELATIVE_EXT"
              mkdir -p "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER"
              cp "$CODESIGNING_FOLDER_PATH/dylib-Info-template.plist" "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/Info.plist"
              plutil -replace CFBundleExecutable -string "$FULL_MODULE_NAME" "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/Info.plist"
              plutil -replace CFBundleIdentifier -string "$FRAMEWORK_BUNDLE_ID" "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/Info.plist"
          fi

          echo "Installing binary for $FRAMEWORK_FOLDER/$FULL_MODULE_NAME"
          mv "$FULL_EXT" "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/$FULL_MODULE_NAME"
          # Create a placeholder .fwork file where the .so was
          echo "$FRAMEWORK_FOLDER/$FULL_MODULE_NAME" > ${FULL_EXT%.so}.fwork
          # Create a back reference to the .so file location in the framework
          echo "${RELATIVE_EXT%.so}.fwork" > "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/$FULL_MODULE_NAME.origin"
       }

       PYTHON_VER=$(ls -1 "$CODESIGNING_FOLDER_PATH/python/lib")
       echo "Install Python $PYTHON_VER standard library extension modules..."
       find "$CODESIGNING_FOLDER_PATH/python/lib/$PYTHON_VER/lib-dynload" -name "*.so" | while read FULL_EXT; do
          install_dylib python/lib/$PYTHON_VER/lib-dynload/ "$FULL_EXT"
       done

       # Clean up dylib template
       rm -f "$CODESIGNING_FOLDER_PATH/dylib-Info-template.plist"

       echo "Signing frameworks as $EXPANDED_CODE_SIGN_IDENTITY_NAME ($EXPANDED_CODE_SIGN_IDENTITY)..."
       find "$CODESIGNING_FOLDER_PATH/Frameworks" -name "*.framework" -exec /usr/bin/codesign --force --sign "$EXPANDED_CODE_SIGN_IDENTITY" ${OTHER_CODE_SIGN_FLAGS:-} -o runtime --timestamp=none --preserve-metadata=identifier,entitlements,flags --generate-entitlement-der "{}" \;

10. Add Objective C code to initialize and use a Python interpreter in
    embedded mode. You should ensure that:

   * "UTF-8 mode" is *enabled*;

   * "Buffered stdio" is *disabled*;

   * "Writing bytecode" is *disabled*;

   * "Signal handlers" are *enabled*;

   * "PYTHONHOME" for the interpreter is configured to point at the
     "python" subfolder of your app's bundle; and

   * The "PYTHONPATH" for the interpreter includes:

     * the "python/lib/python3.X" subfolder of your app's bundle,

     * the "python/lib/python3.X/lib-dynload" subfolder of your app's
       bundle, and

     * the "app" subfolder of your app's bundle

   Your app's bundle location can be determined using "[[NSBundle
   mainBundle] resourcePath]".

Steps 8, 9 and 10 of these instructions assume that you have a single
folder of pure Python application code, named "app". If you have
third-party binary modules in your app, some additional steps will be
required:

* You need to ensure that any folders containing third-party binaries
  are either associated with the app target, or copied in as part of
  step 8. Step 8 should also purge any binaries that are not
  appropriate for the platform a specific build is targeting (i.e.,
  delete any device binaries if you're building an app targeting the
  simulator).

* Any folders that contain third-party binaries must be processed into
  framework form by step 9. The invocation of "install_dylib" that
  processes the "lib-dynload" folder can be copied and adapted for
  this purpose.

* If you're using a separate folder for third-party packages, ensure
  that folder is included as part of the "PYTHONPATH" configuration in
  step 10.
