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The iOS testbed now treats the app_packages folder as a site folder. This ensures it is on the path, but also ensures any .pth files are processed on app startup.
393 lines
19 KiB
ReStructuredText
393 lines
19 KiB
ReStructuredText
.. _using-ios:
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===================
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Using Python on iOS
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===================
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:Authors:
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Russell Keith-Magee (2024-03)
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Python on iOS is unlike Python on desktop platforms. On a desktop platform,
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Python is generally installed as a system resource that can be used by any user
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of that computer. Users then interact with Python by running a :program:`python`
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executable and entering commands at an interactive prompt, or by running a
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Python script.
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On iOS, there is no concept of installing as a system resource. The only unit
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of software distribution is an "app". There is also no console where you could
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run a :program:`python` executable, or interact with a Python REPL.
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As a result, the only way you can use Python on iOS is in embedded mode - that
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is, by writing a native iOS application, and embedding a Python interpreter
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using ``libPython``, and invoking Python code using the :ref:`Python embedding
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API <embedding>`. The full Python interpreter, the standard library, and all
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your Python code is then packaged as a standalone bundle that can be
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distributed via the iOS App Store.
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If you're looking to experiment for the first time with writing an iOS app in
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Python, projects such as `BeeWare <https://beeware.org>`__ and `Kivy
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<https://kivy.org>`__ will provide a much more approachable user experience.
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These projects manage the complexities associated with getting an iOS project
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running, so you only need to deal with the Python code itself.
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Python at runtime on iOS
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========================
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iOS version compatibility
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-------------------------
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The minimum supported iOS version is specified at compile time, using the
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:option:`--host` option to ``configure``. By default, when compiled for iOS,
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Python will be compiled with a minimum supported iOS version of 13.0. To use a
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different minimum iOS version, provide the version number as part of the
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:option:`!--host` argument - for example,
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``--host=arm64-apple-ios15.4-simulator`` would compile an ARM64 simulator build
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with a deployment target of 15.4.
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Platform identification
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-----------------------
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When executing on iOS, ``sys.platform`` will report as ``ios``. This value will
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be returned on an iPhone or iPad, regardless of whether the app is running on
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the simulator or a physical device.
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Information about the specific runtime environment, including the iOS version,
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device model, and whether the device is a simulator, can be obtained using
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:func:`platform.ios_ver`. :func:`platform.system` will report ``iOS`` or
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``iPadOS``, depending on the device.
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:func:`os.uname` reports kernel-level details; it will report a name of
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``Darwin``.
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Standard library availability
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-----------------------------
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The Python standard library has some notable omissions and restrictions on
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iOS. See the :ref:`API availability guide for iOS <mobile-availability>` for
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details.
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Binary extension modules
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------------------------
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One notable difference about iOS as a platform is that App Store distribution
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imposes hard requirements on the packaging of an application. One of these
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requirements governs how binary extension modules are distributed.
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The iOS App Store requires that *all* binary modules in an iOS app must be
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dynamic libraries, contained in a framework with appropriate metadata, stored
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in the ``Frameworks`` folder of the packaged app. There can be only a single
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binary per framework, and there can be no executable binary material outside
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the ``Frameworks`` folder.
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This conflicts with the usual Python approach for distributing binaries, which
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allows a binary extension module to be loaded from any location on
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``sys.path``. To ensure compliance with App Store policies, an iOS project must
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post-process any Python packages, converting ``.so`` binary modules into
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individual standalone frameworks with appropriate metadata and signing. For
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details on how to perform this post-processing, see the guide for :ref:`adding
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Python to your project <adding-ios>`.
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To help Python discover binaries in their new location, the original ``.so``
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file on ``sys.path`` is replaced with a ``.fwork`` file. This file is a text
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file containing the location of the framework binary, relative to the app
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bundle. To allow the framework to resolve back to the original location, the
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framework must contain a ``.origin`` file that contains the location of the
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``.fwork`` file, relative to the app bundle.
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For example, consider the case of an import ``from foo.bar import _whiz``,
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where ``_whiz`` is implemented with the binary module
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``sources/foo/bar/_whiz.abi3.so``, with ``sources`` being the location
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registered on ``sys.path``, relative to the application bundle. This module
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*must* be distributed as ``Frameworks/foo.bar._whiz.framework/foo.bar._whiz``
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(creating the framework name from the full import path of the module), with an
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``Info.plist`` file in the ``.framework`` directory identifying the binary as a
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framework. The ``foo.bar._whiz`` module would be represented in the original
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location with a ``sources/foo/bar/_whiz.abi3.fwork`` marker file, containing
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the path ``Frameworks/foo.bar._whiz/foo.bar._whiz``. The framework would also
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contain ``Frameworks/foo.bar._whiz.framework/foo.bar._whiz.origin``, containing
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the path to the ``.fwork`` file.
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When running on iOS, the Python interpreter will install an
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:class:`~importlib.machinery.AppleFrameworkLoader` that is able to read and
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import ``.fwork`` files. Once imported, the ``__file__`` attribute of the
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binary module will report as the location of the ``.fwork`` file. However, the
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:class:`~importlib.machinery.ModuleSpec` for the loaded module will report the
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``origin`` as the location of the binary in the framework folder.
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Compiler stub binaries
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----------------------
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Xcode doesn't expose explicit compilers for iOS; instead, it uses an ``xcrun``
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script that resolves to a full compiler path (e.g., ``xcrun --sdk iphoneos
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clang`` to get the ``clang`` for an iPhone device). However, using this script
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poses two problems:
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* The output of ``xcrun`` includes paths that are machine specific, resulting
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in a sysconfig module that cannot be shared between users; and
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* It results in ``CC``/``CPP``/``LD``/``AR`` definitions that include spaces.
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There is a lot of C ecosystem tooling that assumes that you can split a
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command line at the first space to get the path to the compiler executable;
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this isn't the case when using ``xcrun``.
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To avoid these problems, Python provided stubs for these tools. These stubs are
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shell script wrappers around the underingly ``xcrun`` tools, distributed in a
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``bin`` folder distributed alongside the compiled iOS framework. These scripts
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are relocatable, and will always resolve to the appropriate local system paths.
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By including these scripts in the bin folder that accompanies a framework, the
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contents of the ``sysconfig`` module becomes useful for end-users to compile
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their own modules. When compiling third-party Python modules for iOS, you
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should ensure these stub binaries are on your path.
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Installing Python on iOS
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========================
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Tools for building iOS apps
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---------------------------
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Building for iOS requires the use of Apple's Xcode tooling. It is strongly
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recommended that you use the most recent stable release of Xcode. This will
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require the use of the most (or second-most) recently released macOS version,
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as Apple does not maintain Xcode for older macOS versions. The Xcode Command
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Line Tools are not sufficient for iOS development; you need a *full* Xcode
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install.
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If you want to run your code on the iOS simulator, you'll also need to install
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an iOS Simulator Platform. You should be prompted to select an iOS Simulator
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Platform when you first run Xcode. Alternatively, you can add an iOS Simulator
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Platform by selecting from the Platforms tab of the Xcode Settings panel.
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.. _adding-ios:
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Adding Python to an iOS project
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-------------------------------
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Python can be added to any iOS project, using either Swift or Objective C. The
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following examples will use Objective C; if you are using Swift, you may find a
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library like `PythonKit <https://github.com/pvieito/PythonKit>`__ to be
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helpful.
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To add Python to an iOS Xcode project:
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1. Build or obtain a Python ``XCFramework``. See the instructions in
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:source:`iOS/README.rst` (in the CPython source distribution) for details on
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how to build a Python ``XCFramework``. At a minimum, you will need a build
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that supports ``arm64-apple-ios``, plus one of either
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``arm64-apple-ios-simulator`` or ``x86_64-apple-ios-simulator``.
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2. Drag the ``XCframework`` into your iOS project. In the following
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instructions, we'll assume you've dropped the ``XCframework`` into the root
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of your project; however, you can use any other location that you want by
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adjusting paths as needed.
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3. Drag the ``iOS/Resources/dylib-Info-template.plist`` file into your project,
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and ensure it is associated with the app target.
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4. Add your application code as a folder in your Xcode project. In the
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following instructions, we'll assume that your user code is in a folder
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named ``app`` in the root of your project; you can use any other location by
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adjusting paths as needed. Ensure that this folder is associated with your
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app target.
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5. Select the app target by selecting the root node of your Xcode project, then
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the target name in the sidebar that appears.
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6. In the "General" settings, under "Frameworks, Libraries and Embedded
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Content", add ``Python.xcframework``, with "Embed & Sign" selected.
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7. In the "Build Settings" tab, modify the following:
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- Build Options
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* User Script Sandboxing: No
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* Enable Testability: Yes
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- Search Paths
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* Framework Search Paths: ``$(PROJECT_DIR)``
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* Header Search Paths: ``"$(BUILT_PRODUCTS_DIR)/Python.framework/Headers"``
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- Apple Clang - Warnings - All languages
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* Quoted Include In Framework Header: No
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8. Add a build step that copies the Python standard library into your app. In
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the "Build Phases" tab, add a new "Run Script" build step *before* the
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"Embed Frameworks" step, but *after* the "Copy Bundle Resources" step. Name
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the step "Install Target Specific Python Standard Library", disable the
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"Based on dependency analysis" checkbox, and set the script content to:
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.. code-block:: bash
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set -e
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mkdir -p "$CODESIGNING_FOLDER_PATH/python/lib"
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if [ "$EFFECTIVE_PLATFORM_NAME" = "-iphonesimulator" ]; then
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echo "Installing Python modules for iOS Simulator"
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rsync -au --delete "$PROJECT_DIR/Python.xcframework/ios-arm64_x86_64-simulator/lib/" "$CODESIGNING_FOLDER_PATH/python/lib/"
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else
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echo "Installing Python modules for iOS Device"
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rsync -au --delete "$PROJECT_DIR/Python.xcframework/ios-arm64/lib/" "$CODESIGNING_FOLDER_PATH/python/lib/"
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fi
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Note that the name of the simulator "slice" in the XCframework may be
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different, depending the CPU architectures your ``XCFramework`` supports.
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9. Add a second build step that processes the binary extension modules in the
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standard library into "Framework" format. Add a "Run Script" build step
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*directly after* the one you added in step 8, named "Prepare Python Binary
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Modules". It should also have "Based on dependency analysis" unchecked, with
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the following script content:
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.. code-block:: bash
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set -e
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install_dylib () {
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INSTALL_BASE=$1
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FULL_EXT=$2
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# The name of the extension file
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EXT=$(basename "$FULL_EXT")
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# The location of the extension file, relative to the bundle
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RELATIVE_EXT=${FULL_EXT#$CODESIGNING_FOLDER_PATH/}
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# The path to the extension file, relative to the install base
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PYTHON_EXT=${RELATIVE_EXT/$INSTALL_BASE/}
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# The full dotted name of the extension module, constructed from the file path.
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FULL_MODULE_NAME=$(echo $PYTHON_EXT | cut -d "." -f 1 | tr "/" ".");
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# A bundle identifier; not actually used, but required by Xcode framework packaging
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FRAMEWORK_BUNDLE_ID=$(echo $PRODUCT_BUNDLE_IDENTIFIER.$FULL_MODULE_NAME | tr "_" "-")
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# The name of the framework folder.
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FRAMEWORK_FOLDER="Frameworks/$FULL_MODULE_NAME.framework"
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# If the framework folder doesn't exist, create it.
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if [ ! -d "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER" ]; then
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echo "Creating framework for $RELATIVE_EXT"
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mkdir -p "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER"
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cp "$CODESIGNING_FOLDER_PATH/dylib-Info-template.plist" "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/Info.plist"
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plutil -replace CFBundleExecutable -string "$FULL_MODULE_NAME" "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/Info.plist"
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plutil -replace CFBundleIdentifier -string "$FRAMEWORK_BUNDLE_ID" "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/Info.plist"
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fi
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echo "Installing binary for $FRAMEWORK_FOLDER/$FULL_MODULE_NAME"
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mv "$FULL_EXT" "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/$FULL_MODULE_NAME"
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# Create a placeholder .fwork file where the .so was
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echo "$FRAMEWORK_FOLDER/$FULL_MODULE_NAME" > ${FULL_EXT%.so}.fwork
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# Create a back reference to the .so file location in the framework
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echo "${RELATIVE_EXT%.so}.fwork" > "$CODESIGNING_FOLDER_PATH/$FRAMEWORK_FOLDER/$FULL_MODULE_NAME.origin"
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}
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PYTHON_VER=$(ls -1 "$CODESIGNING_FOLDER_PATH/python/lib")
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echo "Install Python $PYTHON_VER standard library extension modules..."
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find "$CODESIGNING_FOLDER_PATH/python/lib/$PYTHON_VER/lib-dynload" -name "*.so" | while read FULL_EXT; do
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install_dylib python/lib/$PYTHON_VER/lib-dynload/ "$FULL_EXT"
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done
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# Clean up dylib template
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rm -f "$CODESIGNING_FOLDER_PATH/dylib-Info-template.plist"
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echo "Signing frameworks as $EXPANDED_CODE_SIGN_IDENTITY_NAME ($EXPANDED_CODE_SIGN_IDENTITY)..."
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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 "{}" \;
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10. Add Objective C code to initialize and use a Python interpreter in embedded
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mode. You should ensure that:
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* UTF-8 mode (:c:member:`PyPreConfig.utf8_mode`) is *enabled*;
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* Buffered stdio (:c:member:`PyConfig.buffered_stdio`) is *disabled*;
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* Writing bytecode (:c:member:`PyConfig.write_bytecode`) is *disabled*;
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* Signal handlers (:c:member:`PyConfig.install_signal_handlers`) are *enabled*;
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* System logging (:c:member:`PyConfig.use_system_logger`) is *enabled*
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(optional, but strongly recommended; this is enabled by default);
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* :envvar:`PYTHONHOME` for the interpreter is configured to point at the
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``python`` subfolder of your app's bundle; and
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* The :envvar:`PYTHONPATH` for the interpreter includes:
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- the ``python/lib/python3.X`` subfolder of your app's bundle,
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- the ``python/lib/python3.X/lib-dynload`` subfolder of your app's bundle, and
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- the ``app`` subfolder of your app's bundle
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Your app's bundle location can be determined using ``[[NSBundle mainBundle]
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resourcePath]``.
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Steps 8, 9 and 10 of these instructions assume that you have a single folder of
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pure Python application code, named ``app``. If you have third-party binary
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modules in your app, some additional steps will be required:
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* You need to ensure that any folders containing third-party binaries are
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either associated with the app target, or copied in as part of step 8. Step 8
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should also purge any binaries that are not appropriate for the platform a
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specific build is targeting (i.e., delete any device binaries if you're
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building an app targeting the simulator).
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* Any folders that contain third-party binaries must be processed into
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framework form by step 9. The invocation of ``install_dylib`` that processes
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the ``lib-dynload`` folder can be copied and adapted for this purpose.
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* If you're using a separate folder for third-party packages, ensure that folder
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is included as part of the :envvar:`PYTHONPATH` configuration in step 10.
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* If any of the folders that contain third-party packages will contain ``.pth``
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files, you should add that folder as a *site directory* (using
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:meth:`site.addsitedir`), rather than adding to :envvar:`PYTHONPATH` or
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:attr:`sys.path` directly.
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Testing a Python package
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------------------------
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The CPython source tree contains :source:`a testbed project <iOS/testbed>` that
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is used to run the CPython test suite on the iOS simulator. This testbed can also
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be used as a testbed project for running your Python library's test suite on iOS.
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After building or obtaining an iOS XCFramework (See :source:`iOS/README.rst`
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for details), create a clone of the Python iOS testbed project by running:
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.. code-block:: bash
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$ python iOS/testbed clone --framework <path/to/Python.xcframework> --app <path/to/module1> --app <path/to/module2> app-testbed
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You will need to modify the ``iOS/testbed`` reference to point to that
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directory in the CPython source tree; any folders specified with the ``--app``
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flag will be copied into the cloned testbed project. The resulting testbed will
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be created in the ``app-testbed`` folder. In this example, the ``module1`` and
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``module2`` would be importable modules at runtime. If your project has
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additional dependencies, they can be installed into the
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``app-testbed/iOSTestbed/app_packages`` folder (using ``pip install --target
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app-testbed/iOSTestbed/app_packages`` or similar).
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You can then use the ``app-testbed`` folder to run the test suite for your app,
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For example, if ``module1.tests`` was the entry point to your test suite, you
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could run:
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.. code-block:: bash
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$ python app-testbed run -- module1.tests
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This is the equivalent of running ``python -m module1.tests`` on a desktop
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Python build. Any arguments after the ``--`` will be passed to the testbed as
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if they were arguments to ``python -m`` on a desktop machine.
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You can also open the testbed project in Xcode by running:
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.. code-block:: bash
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$ open app-testbed/iOSTestbed.xcodeproj
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This will allow you to use the full Xcode suite of tools for debugging.
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App Store Compliance
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====================
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The only mechanism for distributing apps to third-party iOS devices is to
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submit the app to the iOS App Store; apps submitted for distribution must pass
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Apple's app review process. This process includes a set of automated validation
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rules that inspect the submitted application bundle for problematic code.
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The Python standard library contains some code that is known to violate these
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automated rules. While these violations appear to be false positives, Apple's
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review rules cannot be challenged; so, it is necessary to modify the Python
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standard library for an app to pass App Store review.
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The Python source tree contains
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:source:`a patch file <Mac/Resources/app-store-compliance.patch>` that will remove
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all code that is known to cause issues with the App Store review process. This
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patch is applied automatically when building for iOS.
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