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Integrating Python into Scala Stack

Integrating various python machine learning libraries in scala stack

This post will walk you through integrating python machine-learning libraries into your existing scala stack.

Before we proceed further, let us answer the basic question:

Why not use scala native libraries?

  • Primarily, there are a very limited set of options available in Scala, spark mllib, deeplearning4j.

  • The python ML ecosystem is huge, scikit-learn itself is enough for most usecases. With the recent surge of deep-learning, most of the established and stable libraries such as Keras/Tensorflow/Theano are available in python.

Our goal is to get the best of both worlds: Scala’s strictly typed system and Python’s plethora of libraries at our disposal

from python import x

While trying to approach this problem, we investigated various libraries. Here is basic case study regarding each one of them:

  • Jython: Jython is basically a re-implementation of python in Java. Although it does include most of the python modules, it lacks the support for C-Extension modules. Which essentially renders Jython useless for most of the libraries in the ML ecosystem as all of them use C-Extension to speedup the processing.

  • JyNI: (Jython Native Interface) JyNI is a compatibility layer with the goal to enable Jython to use native CPython extensions like NumPy or SciPy. However, JyNI doesn’t currently support the entire Python C-API, so it is not currently at a state where we can use it for libraries built using Cython.

  • Jep: (Java Embeded Python) Jep takes a different route and embeds CPython in Java using JNI. Long story short, if you need to include CPython modules (such as numpy) Jep is the way to go.

How Jep Works?

Jep uses JNI and the CPython API to start up the Python interpreter inside the JVM. When you create a Jep instance in Java, a sub-interpreter will be created for that Jep instance and will remain in memory until the Jep instance is closed with jep.close(). Have a look at Jep’s documentation for further details.

Let us demonstrate via a simple example:

# Define this in file ``
def add(a, b):
	return a + b
object JepAddExample extends App {
	val jep = new Jep()
	val a = 2
	val b = 3
	// There are multiple ways to evaluate. Let us demonstrate them:
	jep.eval(s"c = add($a, $b)")
	val ans = jep.getValue("c").asInstanceOf[Int]
	val ans2 = jep.invoke("add", a, b).asInstanceOf[Int]


import keras {via scala}

It’s about time we experience the magic of keras from scala land. This example can be found on

We will demonstrate how to train a simple covnet on the MNIST dataset via scala. This is a sample example from keras

Trains a simple convnet on the MNIST dataset.

Gets to 99.25% test accuracy after 12 epochs
(there is still a lot of margin for parameter tuning).
16 seconds per epoch on a GRID K520 GPU.

from __future__ import print_function
import numpy as np
np.random.seed(1337)  # for reproducibility

from keras.datasets import mnist
from keras.models import Sequential
from keras.layers import Dense, Dropout, Activation, Flatten
from keras.layers import Convolution2D, MaxPooling2D
from keras.utils import np_utils

batch_size = 128
nb_classes = 10
nb_epoch = 12

# input image dimensions
img_rows, img_cols = 28, 28
# number of convolutional filters to use
nb_filters = 32
# size of pooling area for max pooling
nb_pool = 2
# convolution kernel size
nb_conv = 3

# the data, shuffled and split between train and test sets
(X_train, y_train), (X_test, y_test) = mnist.load_data()

X_train = X_train.reshape(X_train.shape[0], 1, img_rows, img_cols)
X_test = X_test.reshape(X_test.shape[0], 1, img_rows, img_cols)
X_train = X_train.astype('float32')
X_test = X_test.astype('float32')
X_train /= 255
X_test /= 255
print('X_train shape:', X_train.shape)
print(X_train.shape[0], 'train samples')
print(X_test.shape[0], 'test samples')

# convert class vectors to binary class matrices
Y_train = np_utils.to_categorical(y_train, nb_classes)
Y_test = np_utils.to_categorical(y_test, nb_classes)

model = Sequential()

model.add(Convolution2D(nb_filters, nb_conv, nb_conv,
                        input_shape=(1, img_rows, img_cols)))
model.add(Convolution2D(nb_filters, nb_conv, nb_conv))
model.add(MaxPooling2D(pool_size=(nb_pool, nb_pool)))


              metrics=['accuracy']), Y_train, batch_size=batch_size, nb_epoch=nb_epoch,
          verbose=1, validation_data=(X_test, Y_test))
score = model.evaluate(X_test, Y_test, verbose=0)
print('Test score:', score[0])
print('Test accuracy:', score[1])
object KerasScala extends App {
	val jep = new Jep()
	val score = jep.getValue("score[0]").asInstanceOf[Double]
	val accuracy = jep.getValue("score[1]").asInstanceOf[Double]
	println(s"score is $score and accuracy is $accuracy")

With this we’ve successfully imported keras in Scala. The possibilities with this are endless.

For instance, you can train a model in an offline fashion using python itself and later use Scala (alongwith Spark) to run the model over a very large dataset in a multi-threaded environment.

And also import anitgravity

Python XKCD

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Sushant Hiray - Foodie. Coder. Reader. Binge Watching.
Open Source Evangelist