# Python for Android

If you obtained this README as part of a release package, then the only
applicable sections are "Prerequisites", "Testing", and "Using in your own app".

If you obtained this README as part of the CPython source tree, then you can
also follow the other sections to compile Python for Android yourself.

However, most app developers should not need to do any of these things manually.
Instead, use one of the tools listed
[here](https://docs.python.org/3/using/android.html), which will provide a much
easier experience.


## Prerequisites

If you already have an Android SDK installed, export the `ANDROID_HOME`
environment variable to point at its location. Otherwise, here's how to install
it:

* Download the "Command line tools" from <https://developer.android.com/studio>.
* Create a directory `android-sdk/cmdline-tools`, and unzip the command line
  tools package into it.
* Rename `android-sdk/cmdline-tools/cmdline-tools` to
  `android-sdk/cmdline-tools/latest`.
* `export ANDROID_HOME=/path/to/android-sdk`

The `android.py` script will automatically use the SDK's `sdkmanager` to install
any packages it needs.

The script also requires the following commands to be on the `PATH`:

* `curl`
* `java` (or set the `JAVA_HOME` environment variable)


## Building

Python can be built for Android on any POSIX platform supported by the Android
development tools, which currently means Linux or macOS.

First we'll make a "build" Python (for your development machine), then use it to
help produce a "host" Python for Android. So make sure you have all the usual
tools and libraries needed to build Python for your development machine.

The easiest way to do a build is to use the `android.py` script. You can either
have it perform the entire build process from start to finish in one step, or
you can do it in discrete steps that mirror running `configure` and `make` for
each of the two builds of Python you end up producing.

The discrete steps for building via `android.py` are:

```sh
./android.py configure-build
./android.py make-build
./android.py configure-host HOST
./android.py make-host HOST
```

`HOST` identifies which architecture to build. To see the possible values, run
`./android.py configure-host --help`.

To do all steps in a single command, run:

```sh
./android.py build HOST
```

In the end you should have a build Python in `cross-build/build`, and a host
Python in `cross-build/HOST`.

You can use `--` as a separator for any of the `configure`-related commands –
including `build` itself – to pass arguments to the underlying `configure`
call. For example, if you want a pydebug build that also caches the results from
`configure`, you can do:

```sh
./android.py build HOST -- -C --with-pydebug
```


## Packaging

After building an architecture as described in the section above, you can
package it for release with this command:

```sh
./android.py package HOST
```

`HOST` is defined in the section above.

This will generate a tarball in `cross-build/HOST/dist`, whose structure is
similar to the `Android` directory of the CPython source tree.


## Testing

The Python test suite can be run on Linux, macOS, or Windows.

On Linux, the emulator needs access to the KVM virtualization interface. This may
require adding your user to a group, or changing your udev rules. On GitHub
Actions, the test script will do this automatically using the commands shown
[here](https://github.blog/changelog/2024-04-02-github-actions-hardware-accelerated-android-virtualization-now-available/).

The test suite can usually be run on a device with 2 GB of RAM, but this is
borderline, so you may need to increase it to 4 GB. As of Android
Studio Koala, 2 GB is the default for all emulators, although the user interface
may indicate otherwise. Locate the emulator's directory under `~/.android/avd`,
and find `hw.ramSize` in both config.ini and hardware-qemu.ini. Either set these
manually to the same value, or use the Android Studio Device Manager, which will
update both files.

You can run the test suite either:

* Within the CPython repository, after doing a build as described above. On
  Windows, you won't be able to do the build on the same machine, so you'll have
  to copy the `cross-build/HOST/prefix` directory from somewhere else.

* Or by taking a release package built using the `package` command, extracting
  it wherever you want, and using its own copy of `android.py`.

The test script supports the following modes:

* In `--connected` mode, it runs on a device or emulator you have already
  connected to the build machine. List the available devices with
  `$ANDROID_HOME/platform-tools/adb devices -l`, then pass a device ID to the
  script like this:

  ```sh
  ./android.py test --connected emulator-5554
  ```

* In `--managed` mode, it uses a temporary headless emulator defined in the
  `managedDevices` section of testbed/app/build.gradle.kts. This mode is slower,
  but more reproducible.

  We currently define two devices: `minVersion` and `maxVersion`, corresponding
  to our minimum and maximum supported Android versions. For example:

  ```sh
  ./android.py test --managed maxVersion
  ```

By default, the only messages the script will show are Python's own stdout and
stderr. Add the `-v` option to also show Gradle output, and non-Python logcat
messages.

Any other arguments on the `android.py test` command line will be passed through
to `python -m test` – use `--` to separate them from android.py's own options.
See the [Python Developer's
Guide](https://devguide.python.org/testing/run-write-tests/) for common options
– most of them will work on Android, except for those that involve subprocesses,
such as `-j`.

Every time you run `android.py test`, changes in pure-Python files in the
repository's `Lib` directory will be picked up immediately. Changes in C files,
and architecture-specific files such as sysconfigdata, will not take effect
until you re-run `android.py make-host` or `build`.

The testbed app can also be used to test third-party packages. For more details,
run `android.py test --help`, paying attention to the options `--site-packages`,
`--cwd`, `-c` and `-m`.


## Using in your own app

See https://docs.python.org/3/using/android.html.