Experimental feature in GraalVM


Since GraalVM supports several other programming languages including JavaScript, R, Ruby, and those that compile to LLVM bitcode, it also provides a Python API to interact with them. In fact, GraalVM uses this API internally to execute Python C extensions using the GraalVM LLVM runtime.

You can import the polyglot module to interact with other languages:

import polyglot

You can import a global value from the entire polyglot scope:

imported_polyglot_global = polyglot.import_value("global_name")

This global value should then work as expected:

  • Accessing attributes assumes it reads from the members namespace.
  • Accessing items is supported both with strings and numbers.
  • Calling methods on the result tries to do a straight invoke and falls back to reading the member and trying to execute it.

You can evaluate some inlined code from another language:

polyglot.eval(string="1 + 1", language="ruby")

You can evaluate some code from a file, by passing the path to it:

polyglot.eval(path="./my_ruby_file.rb", language="ruby")

If you pass a file, you can also rely on the file-based language detection:


You can export some oblect from Python to other supported languages so they can import it:

foo = object()
polyglot.export_value(foo, name="python_foo")

The export function can be used as a decorator. In this case the function name is used as the globally exported name:

def python_method():
    return "Hello from Python!"

Here is an example of how to use the JavaScript regular expression engine to match Python strings. Save this code to the polyglot_example.py file:

import polyglot

re = polyglot.eval(string="RegExp()", language="js")

pattern = re.compile(".*(?:we have (?:a )?matching strings?(?:[!\\?] )?)(.*)")

if pattern.exec("This string does not match"):
    raise SystemError("that shouldn't happen")

md = pattern.exec("Look, we have matching strings! This string was matched by Graal.js")
if not md:
    raise SystemError("this should have matched")

print("Here is what we found: '%s'" % md[1])

To run it, pass the --jvm --polyglot option to the graalpython launcher:

graalpython --jvm --polyglot polyglot_example.py

This program matches Python strings using the JavaScript regular expression object. Python reads the captured group from the JavaScript result and prints: Here is what we found: ‘This string was matched by Graal.js’.

As a more complex example, see how you can read a file using R, process the data in Python, and use R again to display the resulting data image, using both the R and Python libraries in conjunction. To run this example, first install the required R library:

R -e 'install.packages("https://www.rforge.net/src/contrib/jpeg_0.1-8.tar.gz", repos=NULL)'

This example also uses image_magix.py and works on a JPEG image input (you can try with this image). These files have to be in the same folder that the script below is located in and executed from.

import polyglot
import sys
import time
sys.path.insert(0, ".")
from image_magix import Image

load_jpeg = polyglot.eval(string="""function(file.name) {
    jimg <- readJPEG(file.name)
    jimg <- jimg*255
}""", language="R")

raw_data = load_jpeg("python_demo_picture.jpg")

# the dimensions are R attributes; define function to access them
getDim = polyglot.eval(string="function(v, pos) dim(v)[[pos]]", language="R")

# Create object of Python class 'Image' with loaded JPEG data
image = Image(getDim(raw_data, 2), getDim(raw_data, 1), raw_data)

# Run Sobel filter
result = image.sobel()

draw = polyglot.eval(string="""function(processedImgObj) {
    mx <- matrix(processedImgObj$`@data`/255, nrow=processedImgObj$`@height`, ncol=processedImgObj$`@width`)
    grid.raster(mx, height=unit(nrow(mx),"points"))
}""", language="R")


Java Interoperability #

Finally, to interoperate with Java (only when running on the JVM), you can use the java module:

import java
BigInteger = java.type("java.math.BigInteger")
myBigInt = BigInteger(42)
# public Java methods can just be called
# Java method names that are keywords in Python can be accessed using "[]"
byteArray = myBigInt.toByteArray()
# Java arrays can act like Python lists

For packages under the java package, you can also use the normal Python import syntax:

import java.util.ArrayList
from java.util import ArrayList

java.util.ArrayList == ArrayList

al = ArrayList()
# prints [1, 12]

In addition to the type builtin method, the java module exposes the following methods as well:

Builtin Specification
instanceof(obj, class) returns True if obj is an instance of class (class must be a foreign object class)
is_function(obj) returns True if obj is a Java host language function wrapped using Truffle interop
is_object(obj) returns True if obj if the argument is Java host language object wrapped using Truffle interop
is_symbol(obj) returns True if obj if the argument is a Java host symbol, representing the constructor and static members of a Java class, as obtained by java.type
import java
ArrayList = java.type('java.util.ArrayList')
my_list = ArrayList()
# prints True
# prints False, my_list is not a Java host symbol
# prints True, symbols are also host objects
# prints True, the add method of ArrayList
print(java.instanceof(my_list, ArrayList))
# prints True

See Polyglot Programming and Embed Languages for more information about interoperability with other programming languages.