Best way to combine Python and Java?
Where to start with learning how to use API's?
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Coming from Java to Python
You're going to have to unlearn a whole bunch of bullshit boilerplate.
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Why not use Jython? The only downside I can immediately think of is if your library uses CPython native extensions.
EDIT: If you can use Jython now but think you may have problems with a later version of the library, I suggest you try to isolate the library from your app (e.g. some sort of adapter interface). Go with the simplest thing that works for the moment, then consider JNI/CPython/etc if and when you ever need to. There's little to be gained by going the (painful) JNI route unless you really have to.
Frankly most ways to somehow run Python directly from within JVM don't work. They are either not-quite-compatible (new release of your third party library can use python 2.6 features and will not work with Jython 2.5) or hacky (it will break with cryptic JVM stacktrace not really leading to solution).
My preferred way to integrate the two would use RPC. XML RPC is not a bad choice here, if you have moderate amounts of data. It is pretty well supported — Python has it in its standard library. Java libraries are also easy to find. Now depending on your setup either Java or Python part would be a server accepting connection from other language.
A less popular but worth considering alternative way to do RPCs is Google protobuffers, which have 2/3 of support for nice rpc. You just need to provide your transport layer. Not that much work and the convenience of writing is reasonable.
Another option is to write a C wrapper around that pieces of Python functionality that you need to expose to Java and use it via JVM native plugins. You can ease the pain by going with SWIG SWIG.
Essentially in your case it works like that:
- Create a SWIG interface for all method calls from Java to C++.
- Create C/C++ code that will receive your calls and internally call python interpreter with right params.
- Convert response you get from python and send it via swig back to your Java code.
This solution is fairly complex, a bit of an overkill in most cases. Still it is worth doing if you (for some reason) cannot afford RPCs. RPC still would be my preferred choice, though.
My project uses some packages that are available only in Python and heavily rely on C libraries. The project also greatly benefits from Java libraries and the JVM. What's the optimal way to call Python functions from Java?
I tried:
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Small web-services: overhead to serialize data, start and stop the services. Also debugging is harder and implementing each new function is now double the effort.
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Jpy: a library that runs an interpreter in the JVM. Spare the service start/stop, but: isn't really feasible for more than a single-liner, data translation between Java and Python is cumbersome, and I also encountered runtime segmentation fault errors.
Any other options?
The project is in the machine-learning domain, so involves exchanging large numeric arrays and text. In some cases the execution switches back and forth between the platforms.