PythonInterpreter interpreter =newPythonInterpreter();
interpreter.exec("import sys\nsys.path.append('pathToModules if they are not there by default')\nimport yourModule");// execute a function that takes a string and returns a stringPyObject someFunc = interpreter.get("funcName");PyObject result = someFunc.__call__(newPyString("Test!"));String realResult =(String) result.__tojava__(String.class);
You can easily call python functions from Java code with Jython. That is as long as your python code itself runs under jython, i.e. doesn’t use some c-extensions that aren’t supported.
If that works for you, it’s certainly the simplest solution you can get. Otherwise you can use org.python.util.PythonInterpreter from the new Java6 interpreter support.
A simple example from the top of my head – but should work I hope: (no error checking done for brevity)
PythonInterpreter interpreter = new PythonInterpreter();
interpreter.exec("import sys\nsys.path.append('pathToModules if they are not there by default')\nimport yourModule");
// execute a function that takes a string and returns a string
PyObject someFunc = interpreter.get("funcName");
PyObject result = someFunc.__call__(new PyString("Test!"));
String realResult = (String) result.__tojava__(String.class);
Hey I thought I would enter my answer to this even though its late. I think there are some important things to consider first with how strong you wish to have the linking between java and python.
Firstly Do you only want to call functions or do you actually want python code to change the data in your java objects? This is very important. If you only want to call some python code with or without arguments, then that is not very difficult. If your arguments are primitives it makes it even more easy. However if you want to have java class implement member functions in python, which change the data of the java object, then this is not so easy or straight forward.
Secondly are we talking cpython or will jython do? I would say cpython is where its at! I would advocate this is why python is so kool! Having such high abstractions however access to c,c++ when needed. Imagine if you could have that in java. This question is not even worth asking if jython is ok because then it is easy anyway.
So I have played with the following methods, and listed them from easy to difficult:
Java to Jython
Advantages: Trivially easy. Have actual references to java objects
Disadvantages: No CPython, Extremely Slow!
Jython from java is so easy, and if this is really enough then great. However it is very slow and no cpython! Is life worth living without cpython I don’t think so! You can easily have python code implementing your member functions for you java objects.
Java to Jython to CPython via Pyro
Pyro is the remote object module for python. You have some object on a cpython interpreter, and you can send it objects which are transferred via serialization and it can also return objects via this method. Note that if you send a serialized python object from jython and then call some functions which change the data in its members, then you will not see those changes in java. You just need to remember to send back the data which you want from pyro. This I believe is the easiest way to get to cpython! You do not need any jni or jna or swig or …. You don’t need to know any c, or c++. kool huh?
Advantages: Access to cpython, not as difficult as following methods
Disadvantages: Cannot change the member data of java objects directly from python. Is somewhat indirect, (jython is middle man).
Java to C/C++ via JNI/JNA/SWIG to Python via Embedded interpreter (maybe using BOOST Libraries?)
OMG this method is not for the faint of heart. And I can tell you it has taken me very long to achieve this in with a decent method. Main reason you would want to do this is so that you can run cpython code which as full rein over you java object. There are major major things to consider before deciding to try and bread java (which is like a chimp) with python (which is like a horse). Firstly if you crash the interpreter that’s lights out for you program! And don’t get me started on concurrency issues! In addition, there is allot allot of boiler, I believe I have found the best configuration to minimize this boiler but still it is allot! So how to go about this:
Consider that C++ is your middle man, your objects are actually c++ objects! Good that you know that now. Just write your object as if your program as in cpp not java, with the data you want to access from both worlds. Then you can use the wrapper generator called swig (http://www.swig.org/Doc1.3/Java.html) to make this accessible to java and compile a dll which you call System.load(dll name here) in java. Get this working first, then move on to the hard part!
To get to python you need to embed an interpreter. Firstly I suggest doing some hello interpreter programs or this tutorial Embedding python in C/C. Once you have that working, its time to make the horse and the monkey dance! You can send you c++ object to python via [boost][3] . I know I have not given you the fish, merely told you where to find the fish. Some pointers to note for this when compiling.
When you compile boost you will need to compile a shared library. And you need to include and link to the stuff you need from jdk, ie jawt.lib, jvm.lib, (you will also need the client jvm.dll in your path when launching the application) As well as the python27.lib or whatever and the boost_python-vc100-mt-1_55.lib.
Then include Python/include, jdk/include, boost and only use shared libraries (dlls) otherwise boost has a teary. And yeah full on I know. There are so many ways in which this can go sour. So make sure you get each thing done block by block. Then put them together.
It’s not smart to have python code inside java. Wrap your python code with flask or other web framework to make it as a microservice. Make your java program able to call this microservice (e.g. via REST).
Beleive me, this is much simple and will save you tons of issues. And the codes are loosely coupled so they are scalable.
Several of the answers mention that you can use JNI or JNA to access cpython but I would not recommend starting from scratch because there are already open source libraries for accessing cpython from java. For example:
Here a library that lets you write your python scripts once and decide which integration method (Jython, CPython/PyPy via Jep and Py4j) to use at runtime:
It depends on what do you mean by python functions? if they were written in cpython you can not directly call them you will have to use JNI, but if they were written in Jython you can easily call them from java, as jython ultimately generates java byte code.
Now when I say written in cpython or jython it doesn’t make much sense because python is python and most code will run on both implementations unless you are using specific libraries which relies on cpython or java.
Depending on your requirements, options like XML-RPC could be useful, which can be used to remotely call functions virtually in any language supporting the protocol.
GraalVM is a good choice. I’ve done Java+Javascript combination with GraalVM for microservice design (Java with Javascript reflection). They recently added support for python, I’d give it a try especially with how big its community has grown over the years.
There are a number of differences. First, Jython programs cannot use CPython
extension modules written in C. These modules usually have files with the
extension .so, .pyd or .dll. If you want to use such a module, you should look
for an equivalent written in pure Python or Java. Although it is technically
feasible to support such extensions – IronPython does so – there are no plans
to do so in Jython.
This gives a pretty good overview over the current options. Some of which are named in other answers. Jython is not usable until they decide to implement Python 3.x and many of the other projects are coming form the python side and want to access java. But there are a few options still, to name something which has not been named yet: gRPC
A class is free to implement
comparison any way it chooses, and it
can choose to make comparison against
None mean something (which actually
makes sense; if someone told you to
implement the None object from
scratch, how else would you get it to
compare True against itself?).
Practically-speaking, there is not much difference since custom comparison operators are rare. But you should use is None as a general rule.
import numpy as np
a = np.zeros(3)# now a is array([0., 0., 0.])
a ==None#compares elementwise, outputs array([False, False, False]), i.e. not boolean!!!
a isNone#compares object to object, outputs False
It depends on what you are comparing to None. Some classes have custom comparison methods that treat == None differently from is None.
In particular the output of a == Nonedoes not even have to be boolean !! – a frequent cause of bugs.
For a specific example take a numpy array where the == comparison is implemented elementwise:
import numpy as np
a = np.zeros(3) # now a is array([0., 0., 0.])
a == None #compares elementwise, outputs array([False, False, False]), i.e. not boolean!!!
a is None #compares object to object, outputs False
Pygame is a 2D game engine for Python (on desktop) that is popular with new programmers. The Pygame Subset for Android describes itself as…
…a port of a subset of Pygame functionality to the Android platform. The goal of the project is to allow the creation of Android-specific games, and to ease the porting of games from PC-like platforms to Android.
The examples include a complete game packaged as an APK, which is pretty interesting.
I’ve also open sourced Ignifuga, my 2D Game Engine. It’s Python/SDL based, and it cross compiles for Android. Even if you don’t use it for games, you might get useful ideas from the code or builder utility (named Schafer, after Tim… you know who).
SL4A does what you want. You can easily install it directly onto your device from their site, and do not need root.
It supports a range of languages. Python is the most mature. By default, it uses Python 2.6, but there is a 3.2 port you can use instead. I have used that port for all kinds of things on a Galaxy S2 and it worked fine.
API
SL4A provides a port of their android library for each supported language. The library provides an interface to the underlying Android API through a single Android object.
from android import Android
droid = Android()
droid.ttsSpeak('hello world') # example using the text to speech facade
Each language has pretty much the same API. You can even use the JavaScript API inside webviews.
let droid = new Android();
droid.ttsSpeak("hello from js");
User Interfaces
For user interfaces, you have three options:
You can easily use the generic, native dialogues and menus through the
API. This is good for confirmation dialogues and other basic user inputs.
You can also open a webview from inside a Python script, then use HTML5
for the user interface. When you use webviews from Python, you can pass
messages back and forth, between the webview and the Python process that
spawned it. The UI will not be native, but it is still a good option to
have.
There is some support for native Android user interfaces, but I am not
sure how well it works; I just haven’t ever used it.
You can mix options, so you can have a webview for the main interface, and still use native dialogues.
QPython
There is a third party project named QPython. It builds on SL4A, and throws in some other useful stuff.
QPython gives you a nicer UI to manage your installation, and includes a little, touchscreen code editor, a Python shell, and a PIP shell for package management. They also have a Python 3 port. Both versions are available from the Play Store, free of charge. QPython also bundles libraries from a bunch of Python on Android projects, including Kivy, so it is not just SL4A.
Note that QPython still develop their fork of SL4A (though, not much to be honest). The main SL4A project itself is pretty much dead.
As a Python lover and Android programmer, I’m sad to say this is not a good way to go. There are two problems:
One problem is that there is a lot more than just a programming language to the Android development tools. A lot of the Android graphics involve XML files to configure the display, similar to HTML. The built-in java objects are integrated with this XML layout, and it’s a lot easier than writing your code to go from logic to bitmap.
The other problem is that the G1 (and probably other Android devices for the near future) are not that fast. 200 MHz processors and RAM is very limited. Even in Java, you have to do a decent amount of rewriting-to-avoid-more-object-creation if you want to make your app perfectly smooth. Python is going to be too slow for a while still on mobile devices.
I wanted to add to what @JohnMudd has written about Kivy. It has been years since the situation he described, and Kivy has evolved substantially.
The biggest selling point of Kivy, in my opinion, is its cross-platform compatibility. You can code and test everything using any desktop environment (Windows/*nix etc.), then package your app for a range of different platforms, including Android, iOS, MacOS and Windows (though apps often lack the native look and feel).
With Kivy’s own KV language, you can code and build the GUI interface easily (it’s just like Java XML, but rather than TextView etc., KV has its own ui.widgets for a similar translation), which is in my opinion quite easy to adopt.
Currently Buildozer and python-for-android are the most recommended tools to build and package your apps. I have tried them both and can firmly say that they make building Android apps with Python a breeze. Their guides are well documented too.
iOS is another big selling point of Kivy. You can use the same code base with few changes required via kivy-ios Homebrew tools, although Xcode is required for the build, before running on their devices (AFAIK the iOS Simulator in Xcode currently doesn’t work for the x86-architecture build). There are also some dependency issues which must be manually compiled and fiddled around with in Xcode to have a successful build, but they wouldn’t be too difficult to resolve and people in Kivy Google Group are really helpful too.
With all that being said, users with good Python knowledge should have no problem picking up the basics quickly.
If you are using Kivy for more serious projects, you may find existing modules unsatisfactory. There are some workable solutions though. With the (work in progress) pyjnius for Android, and pyobjus, users can now access Java/Objective-C classes to control some of the native APIs.
Not at the moment and you would be lucky to get Jython to work soon. If you’re planning to start your development now you would be better off with just sticking to Java for now on.
Using SL4A (which has already been mentioned by itself in other answers) you can run a full-blown web2py instance (other python web frameworks are likely candidates as well). SL4A doesn’t allow you to do native UI components (buttons, scroll bars, and the like), but it does support WebViews. A WebView is basically nothing more than a striped down web browser pointed at a fixed address. I believe the native Gmail app uses a WebView instead of going the regular widget route.
This route would have some interesting features:
In the case of most python web frameworks, you could actually develop and test without using an android device or android emulator.
Whatever Python code you end up writing for the phone could also be put on a public webserver with very little (if any) modification.
You could take advantage of all of the crazy web stuff out there: query, HTML5, CSS3, etc.
I use the QPython app. It’s free and includes a code editor, an interactive interpreter and a package manager, allowing you to create and execute Python programs directly on your device.
Python for android is a project to create your own Python distribution including the modules you want, and create an apk including python, libs, and your application.
Chaquopy is a plugin for Android Studio’s Gradle-based build system. It focuses on close integration with the standard Android development tools.
It provides complete APIs to call Java from Python or Python from Java, allowing the developer to use whichever language is best for each component of their app.
It can automatically download PyPI packages and build them into an app, including selected native packages such as NumPy.
It enables full access to all Android APIs from Python, including the native user interface toolkit (example pure-Python activity).
This is a commercial product, but it’s free for open-source use and will always remain that way.
The Scripting Layer for Android, SL4A, is an open source application that allows programs written in a range of interpreted languages to run on Android. It also provides a high level API that allows these programs to interact with the Android device, making it easy to do stuff like accessing sensor data, sending an SMS, rendering user interfaces and so on.
python-for-android is an open source build tool to let you package Python code into standalone android APKs. These can be passed around, installed, or uploaded to marketplaces such as the Play Store just like any other Android app. This tool was originally developed for the Kivy cross-platform graphical framework, but now supports multiple bootstraps and can be easily extended to package other types of Python apps for Android.
BeeWare allows you to write your app in Python and release it on multiple platforms. No need to rewrite the app in multiple programming languages. It means no issues with build tools, environments, compatibility, etc.
The Scripting Layer for Android, SL4A, is an open source application that allows programs written in a range of interpreted languages to run on Android. It also provides a high level API that allows these programs to interact with the Android device, making it easy to do stuff like accessing sensor data, sending an SMS, rendering user interfaces and so on.
You can also check PySide for Android, which is actually Python bindings for the Qt 4.
There’s a platform called PyMob where apps can be written purely in Python and the compiler tool-flow (PyMob) converts them in native source codes for various platforms.
python-for-android is an open source build tool to let you package Python code into standalone android APKs. These can be passed around, installed, or uploaded to marketplaces such as the Play Store just like any other Android app. This tool was originally developed for the Kivy cross-platform graphical framework, but now supports multiple bootstraps and can be easily extended to package other types of Python apps for Android.
BeeWare allows you to write your app in Python and release it on multiple platforms. No need to rewrite the app in multiple programming languages. It means no issues with build tools, environments, compatibility, etc.
It’s an open source project with both Python 2 and Python 3 implementations. You can download the source and the Android .apk files directly from github.
One more option seems to be pyqtdeploy which citing the docs is:
a tool that, in conjunction with other tools provided with Qt, enables
the deployment of PyQt4 and PyQt5 applications written with Python
v2.7 or Python v3.3 or later. It supports deployment to desktop
platforms (Linux, Windows and OS X) and to mobile platforms (iOS and
Android).
According to Deploying PyQt5 application to Android via pyqtdeploy and Qt5 it is actively developed, although it is difficult to find examples of working Android apps or tutorial on how to cross-compile all the required libraries to Android. It is an interesting project to keep in mind though!
Take a look at BeeWare. At the moment of answering this question it is still in early development. It’s aim is to be able to create native apps with Python for all supported operating systems, including Android.
