Call JavaScript Code in Python

Will Pringle — Thu, 03 Aug 2023 15:26:03 +0000

This article introduces the usage of PythonMonkey, a new Python library for running JavaScript code in Python easily and efficiently. Link to PythonMonkey docs.

Running JavaScript code in Python is valuable for a number of reasons such as leveraging the JavaScript ecosystem in Python or testing your JS code using Python to name a couple.

Required Installation

Install PythonMonkey using pip:

$ pip3 install pythonmonkey

Simple Example

Before getting into the details, let's use PythonMonkey to execute some JavaScript code in Python. We'll just do this in the Python repl by typing python3 into the console:

>>> import pythonmonkey as pm
>>> hello = pm.eval(" 'Hello World'.toUpperCase(); ")
>>> print(hello)
'HELLO WORLD'

Loading a JavaScript Module in Python Example

In this example, we'll have two files: my-javascript-module.js and main.py which imports the functions from the JavaScript code:
my-javascript-module.js:

exports.sayHello = () => { console.log('hello, world') };

main.py:

import pythonmonkey as pm
test = pm.require('./my-javascript-module');
test.sayHello() # this prints hello, world

Which outputs:

hello, world

PythonMonkey API for JS/Py Interoperability

Below are the functions available for use in Python:

pythonmonkey.eval(code_string)
- Input: String containing JavaScript Code
- Evaluates JavaScript code within a JavaScript context. Will return an evaluated JavaScript structure.
- It can take an optional second argument for eval options, but we won't use it in this tutorial.
pythonmonkey.require(moduleIdentifier)
- Input: Path to the JavaScript Module or identifier as per CommonJS spec.
- Loads a JavaScript file / module in Python for use. This is the same usage as Node.js' require function.
pythonmonkey.globalThis
- An object that contains the global variables defined in the JavaScript context. For instance, you could define a variable using pm.eval('const a = 4;') and access it in python using pm.globalThis.a.
pythonmonkey.*
- PythonMonkey also exports the JavaScript API as part of the module. For instance, you could use the WebAssembly API in Python through pythonmonkey.WebAssembly or instantiate a new JavaScript Date using pythonmonkey.Date
pythonmonkey.new(js_constructor)
- Input: A JavaScript constructor.
- Since Python lacks a "new" keyword for instantiating Objects but JavaScript needs it, PythonMonkey provides a pythonmonkey.new() function which returns a factory function for the constructor passed in. For instance, my_date = pythonmonkey.new(pythonmonkey.Date)(0)

For full and up to date documentation for PythonMonkey, refer to the PythonMonkey docs: https://docs.pythonmonkey.io/

Advanced Example (using WASM in Python)

PythonMonkey can be used to execute WebAssembly in Python!

We'll read a WebAssembly .wasm file in Python and load it using pythonmonkey.WebAssembly which has a function "factorial()":

import asyncio # we'll use asyncio to deal with an event loop
import pythonmonkey

# we'll put our code in an async python function
async def async_fn():
  # read the factorial.wasm binary file
  file = open('factorial.wasm', 'rb')
  wasm_bytes = bytearray(file.read())

  # instantiate the WebAssembly code
  wasm_fact = await pythonmonkey.WebAssembly.instantiate(wasm_bytes, {})

  # return the "fac" factorial function from the wasm module
  return wasm_fact.instance.exports.fac;

# await the promise which returns the factorial WebAssembly function
factorial = asyncio.run(async_fn())

# execute WebAssembly code in Python!
print(factorial(4)) # this outputs "24.0" since factorial(4) == 24
print(factorial(5)) # this outputs "120.0"
print(factorial(6)) # this outputs "720.0"

This outputs:

24.0
120.0
720.0

For more advanced examples, check out my article on Executing Rust in Python using WebAssembly & PythonMonkey or Calling C functions in Python using WebAssembly.

Conclusion

In conclusion, PythonMonkey is an effective Python library for executing JavaScript in Python. With PythonMonkey you can call JavaScript from Python and vice versa all by using a Python Library. PythonMonkey can also be used to load JavaScript files or modules within Python and even execute WebAssembly code directly in Python.

Check out PythonMonkey's release article and consider giving it a star on the project's GitHub page to show your support!

Enjoy mixing JavaScript with Python using PythonMonkey!