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Getting Started with ScalaPy

ScalaPy makes it easy to use Python libraries from Scala code. With a simple API, automatic conversion between Scala and Python types, and optional static typing, ScalaPy scales from hobby projects to production systems.

Installation#

First, add ScalaPy to your SBT build:

// JVM
libraryDependencies += "me.shadaj" %% "scalapy-core" % "0.5.2"
// Scala Native
libraryDependencies += "me.shadaj" %%% "scalapy-core" % "0.5.2"

You'll then need to add the Python native libraries to your project and configure SBT to run your code in a separate JVM instance, either manually,

fork := true
import scala.sys.process._
lazy val pythonLdFlags = {
val withoutEmbed = "python3-config --ldflags".!!
if (withoutEmbed.contains("-lpython")) {
withoutEmbed.split(' ').map(_.trim).filter(_.nonEmpty).toSeq
} else {
val withEmbed = "python3-config --ldflags --embed".!!
withEmbed.split(' ').map(_.trim).filter(_.nonEmpty).toSeq
}
}
lazy val pythonLibsDir = {
pythonLdFlags.find(_.startsWith("-L")).get.drop("-L".length)
}
javaOptions += s"-Djna.library.path=$pythonLibsDir"

or using the python-native-libs helper library,

First, add python-native-libs to project/plugins.sbt

libraryDependencies += "ai.kien" %% "python-native-libs" % "0.2.2"

Then, in build.sbt,

fork := true
import ai.kien.python.Python
lazy val python = Python("<optional-path-to-a-python-interpreter-executable>")
lazy val javaOpts = python.scalapyProperties.get.map {
case (k, v) => s"""-D$k=$v"""
}.toSeq
javaOptions ++= javaOpts

If you'd like to use Scala Native, follow the instructions there to create a project with Scala Native 0.4.0-M2. Then, add the following additional configuration to your SBT build to link the Python interpreter.

Manually,

lazy val pythonLdFlags = ... // same as above
nativeLinkingOptions ++= pythonLdFlags

Using python-native-libs,

import ai.kien.python.Python
lazy val python = Python("<optional-path-to-a-python-interpreter-executable>")
lazy val pythonLdFlags = python.ldflags.get
nativeLinkingOptions ++= pythonLdFlags

Hello World!#

Now that ScalaPy is installed, let's start with a simple example. ScalaPy offers a dynamically typed API that's great for making quick Python calls with little ceremony. First, we can use the Python len function to calculate the length of a list. Using py.Dynamic.global, you can access any members of the global scope.

import me.shadaj.scalapy.py
import me.shadaj.scalapy.py.SeqConverters
val listLengthPython = py.Dynamic.global.len(List(1, 2, 3).toPythonProxy)
// listLengthPython: py.Dynamic = 3

Here, we took a Scala List, converted it to a Python list, sent it to the len function, and got back a Python number.

To convert Python values back into Scala, we use the .as method and pass in the type we want.

val listLength = listLengthPython.as[Int]
// listLength: Int = 3

Execution#

ScalaPy officially supports Python 3.{7, 8, 9}. If you want to use another version of Python, you should either define the environment variable SCALAPY_PYTHON_LIBRARY

python --version
# Python 3.8.6
export SCALAPY_PYTHON_LIBRARY=python3.8
sbt run

or set the system property scalapy.python.library

sbt -Dscalapy.python.library=python3.8 run

The environment variable takes precedence over the system property.

Virtualenv#

To use ScalaPy with a Python installation inside a virtualenv, set the path to the Python interpreter executable using either the scalapy.python.programname system property

sbt -Dscalapy.python.programname=/Users/example/example-env/bin/python run

or the SCALAPY_PYTHON_PROGRAMNAME environment variable

export SCALAPY_PYTHON_PROGRAMNAME=/Users/example/example-env/bin/python
sbt run

The environment variable takes precedence over the system property.

This variable is used as the input to the Python/C API function Py_SetProgramName. Py_SetProgramName is run prior to Py_Initialize to set the correct paths to Python run-time libraries (prefix, exec_prefix, ...).