cpython/Apple/iOS

Python on iOS README

iOS support is tier 3.

This document provides a quick overview of some iOS specific features in the Python distribution.

These instructions are only needed if you're planning to compile Python for iOS yourself. Most users should not need to do this. If you're looking to experiment with writing an iOS app in Python, tools such as BeeWare's Briefcase and Kivy's Buildozer will provide a much more approachable user experience.

Compilers for building on iOS

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 an open the Platforms tab of the Xcode Settings panel.

Building Python on iOS

ABIs and Architectures

iOS apps can be deployed on physical devices, and on the iOS simulator. Although the API used on these devices is identical, the ABI is different - you need to link against different libraries for an iOS device build (iphoneos) or an iOS simulator build (iphonesimulator).

Apple uses the XCframework format to allow specifying a single dependency that supports multiple ABIs. An XCframework is a wrapper around multiple ABI-specific frameworks that share a common API.

iOS can also support different CPU architectures within each ABI. At present, there is only a single supported architecture on physical devices - ARM64. However, the simulator supports 2 architectures - ARM64 (for running on Apple Silicon machines), and x86_64 (for running on older Intel-based machines).

To support multiple CPU architectures on a single platform, Apple uses a "fat binary" format - a single physical file that contains support for multiple architectures. It is possible to compile and use a "thin" single architecture version of a binary for testing purposes; however, the "thin" binary will not be portable to machines using other architectures.

Building a multi-architecture iOS XCframework

The Apple subfolder of the Python repository acts as a build script that can be used to coordinate the compilation of a complete iOS XCframework. To use it, run::

python Apple build iOS

This will:

• Configure and compile a version of Python to run on the build machine
• Download pre-compiled binary dependencies for each platform
• Configure and build a Python.framework for each required architecture and iOS SDK
• Merge the multiple Python.framework folders into a single Python.xcframework
• Produce a .tar.gz archive in the cross-build/dist folder containing the Python.xcframework, plus a copy of the Testbed app pre-configured to use the XCframework.

The Apple build script has other entry points that will perform the individual parts of the overall build target, plus targets to test the build, clean the cross-build folder of iOS build products, and perform a complete "build and test" CI run. The --clean flag can also be used on individual commands to ensure that a stale build product are removed before building.

Building a single-architecture framework

If you're using the Apple build script, you won't need to build individual frameworks. However, if you do need to manually configure an iOS Python build for a single framework, the following options are available.

iOS specific arguments to configure

• --enable-framework[=DIR]

  This argument specifies the location where the Python.framework will be installed. If DIR is not specified, the framework will be installed into a subdirectory of the iOS/Frameworks folder.

  This argument must be provided when configuring iOS builds. iOS does not support non-framework builds.

• --with-framework-name=NAME

  Specify the name for the Python framework; defaults to Python.

  Note

  Unless you know what you're doing, changing the name of the Python framework on iOS is not advised. If you use this option, you won't be able to run the Apple build script without making significant manual alterations, and you won't be able to use any binary packages unless you compile them yourself using your own framework name.

Building Python for iOS

The Python build system will create a Python.framework that supports a single ABI with a single architecture. Unlike macOS, iOS does not allow a framework to contain non-library content, so the iOS build will produce a bin and lib folder in the same output folder as Python.framework. The lib folder will be needed at runtime to support the Python library.

If you want to use Python in a real iOS project, you need to produce multiple Python.framework builds, one for each ABI and architecture. iOS builds of Python must be constructed as framework builds. To support this, you must provide the --enable-framework flag when configuring the build. The build also requires the use of cross-compilation. The minimal commands for building Python for the ARM64 iOS simulator will look something like:

export PATH="$(pwd)/Apple/iOS/Resources/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Library/Apple/usr/bin"
./configure \
    --enable-framework \
    --host=arm64-apple-ios-simulator \
    --build=arm64-apple-darwin \
    --with-build-python=/path/to/python.exe
make
make install

In this invocation:

• Apple/iOS/Resources/bin has been added to the path, providing some shims for the compilers and linkers needed by the build. Xcode requires the use of xcrun to invoke compiler tooling. However, if xcrun is pre-evaluated and the result passed to configure, these results can embed user- and version-specific paths into the sysconfig data, which limits the portability of the compiled Python. Alternatively, if xcrun is used as the compiler, it requires that compiler variables like CC include spaces, which can cause significant problems with many C configuration systems which assume that CC will be a single executable.

  To work around this problem, the Apple/iOS/Resources/bin folder contains some wrapper scripts that present as simple compilers and linkers, but wrap underlying calls to xcrun. This allows configure to use a CC definition without spaces, and without user- or version-specific paths, while retaining the ability to adapt to the local Xcode install. These scripts are included in the bin directory of an iOS install.

  These scripts will, by default, use the currently active Xcode installation. If you want to use a different Xcode installation, you can use xcode-select to set a new default Xcode globally, or you can use the DEVELOPER_DIR environment variable to specify an Xcode install. The scripts will use the default iphoneos/iphonesimulator SDK version for the select Xcode install; if you want to use a different SDK, you can set the IOS_SDK_VERSION environment variable. (e.g, setting IOS_SDK_VERSION=17.1 would cause the scripts to use the iphoneos17.1 and iphonesimulator17.1 SDKs, regardless of the Xcode default.)

  The path has also been cleared of any user customizations. A common source of bugs is for tools like Homebrew to accidentally leak macOS binaries into an iOS build. Resetting the path to a known "bare bones" value is the easiest way to avoid these problems.

• --host is the architecture and ABI that you want to build, in GNU compiler triple format. This will be one of:

  • arm64-apple-ios for ARM64 iOS devices.
  • arm64-apple-ios-simulator for the iOS simulator running on Apple Silicon devices.
  • x86_64-apple-ios-simulator for the iOS simulator running on Intel devices.

• --build is the GNU compiler triple for the machine that will be running the compiler. This is one of:

  • arm64-apple-darwin for Apple Silicon devices.
  • x86_64-apple-darwin for Intel devices.

• /path/to/python.exe is the path to a Python binary on the machine that will be running the compiler. This is needed because the Python compilation process involves running some Python code. On a normal desktop build of Python, you can compile a python interpreter and then use that interpreter to run Python code. However, the binaries produced for iOS won't run on macOS, so you need to provide an external Python interpreter. This interpreter must be the same version as the Python that is being compiled. To be completely safe, this should be the exact same commit hash. However, the longer a Python release has been stable, the more likely it is that this constraint can be relaxed - the same micro version will often be sufficient.

• The install target for iOS builds is slightly different to other platforms. On most platforms, make install will install the build into the final runtime location. This won't be the case for iOS, as the final runtime location will be on a physical device.

  However, you still need to run the install target for iOS builds, as it performs some final framework assembly steps. The location specified with --enable-framework will be the location where make install will assemble the complete iOS framework. This completed framework can then be copied and relocated as required.

For a full CPython build, you also need to specify the paths to iOS builds of the binary libraries that CPython depends on (such as XZ, LibFFI and OpenSSL). This can be done by defining library specific environment variables (such as LIBLZMA_CFLAGS, LIBLZMA_LIBS), and the --with-openssl configure option. Versions of these libraries pre-compiled for iOS can be found in this repository. LibFFI is especially important, as many parts of the standard library (including the platform, sysconfig and webbrowser modules) require the use of the ctypes module at runtime.

By default, Python will be compiled with an iOS deployment target (i.e., the minimum supported iOS version) of 13.0. To specify a different deployment target, provide the version number as part of the --host argument - for example, --host=arm64-apple-ios15.4-simulator would compile an ARM64 simulator build with a deployment target of 15.4.

Testing Python on iOS

Testing a multi-architecture framework

Once you have a built an XCframework, you can test that framework by running:

$ python Apple test iOS

Testing a single-architecture framework

The Apple/testbed folder that contains an Xcode project that is able to run the Python test suite on Apple platforms. This project converts the Python test suite into a single test case in Xcode's XCTest framework. The single XCTest passes if the test suite passes.

To run the test suite, configure a Python build for an iOS simulator (i.e., --host=arm64-apple-ios-simulator or --host=x86_64-apple-ios-simulator ), specifying a framework build (i.e. --enable-framework). Ensure that your PATH has been configured to include the Apple/iOS/Resources/bin folder and exclude any non-iOS tools, then run:

make all
make install
make testios

This will:

• Build an iOS framework for your chosen architecture;
• Finalize the single-platform framework;
• Make a clean copy of the testbed project;
• Install the Python iOS framework into the copy of the testbed project; and
• Run the test suite on an "entry-level device" simulator (i.e., an iPhone SE, iPhone 16e, or a similar).

On success, the test suite will exit and report successful completion of the test suite. On a 2022 M1 MacBook Pro, the test suite takes approximately 15 minutes to run; a couple of extra minutes is required to compile the testbed project, and then boot and prepare the iOS simulator.

Debugging test failures

Running python Apple test iOS generates a standalone version of the Apple/testbed project, and runs the full test suite. It does this using Apple/testbed itself - the folder is an executable module that can be used to create and run a clone of the testbed project. The standalone version of the testbed will be created in a directory named cross-build/iOS-testbed.<timestamp>.

You can generate your own standalone testbed instance by running:

python cross-build/iOS/testbed clone my-testbed

In this invocation, my-testbed is the name of the folder for the new testbed clone.

If you've built your own XCframework, or you only want to test a single architecture, you can construct a standalone testbed instance by running:

python Apple/testbed clone --platform iOS --framework <path/to/framework> my-testbed

The framework path can be the path path to a Python.xcframework, or the path to a folder that contains a single-platform Python.framework.

You can then use the my-testbed folder to run the Python test suite, passing in any command line arguments you may require. For example, if you're trying to diagnose a failure in the os module, you might run:

python my-testbed run -- test -W test_os

This is the equivalent of running python -m test -W test_os on a desktop Python build. Any arguments after the -- will be passed to testbed as if they were arguments to python -m on a desktop machine.

Testing in Xcode

You can also open the testbed project in Xcode by running:

open my-testbed/iOSTestbed.xcodeproj

This will allow you to use the full Xcode suite of tools for debugging.

The arguments used to run the test suite are defined as part of the test plan. To modify the test plan, select the test plan node of the project tree (it should be the first child of the root node), and select the "Configurations" tab. Modify the "Arguments Passed On Launch" value to change the testing arguments.

The test plan also disables parallel testing, and specifies the use of the Testbed.lldbinit file for providing configuration of the debugger. The default debugger configuration disables automatic breakpoints on the SIGINT, SIGUSR1, SIGUSR2, and SIGXFSZ signals.

Testing on an iOS device

To test on an iOS device, the app needs to be signed with known developer credentials. To obtain these credentials, you must have an iOS Developer account, and your Xcode install will need to be logged into your account (see the Accounts tab of the Preferences dialog).

Once the project is open, and you're signed into your Apple Developer account, select the root node of the project tree (labeled "iOSTestbed"), then the "Signing & Capabilities" tab in the details page. Select a development team (this will likely be your own name), and plug in a physical device to your macOS machine with a USB cable. You should then be able to select your physical device from the list of targets in the pulldown in the Xcode titlebar.