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WebAssembly with Python

  • WebAssembly support for Python is good thanks to CPython on wasm32-wasi.
  • CPython on wasm32-wasi consists of utilities and libraries for building CPython sources for the WebAssembly platform using the WASI SDK and wasix.
  • Python is one of the most desirable languages to work with WebAssembly.
  • The Python interpreter needs to be compiled to Wasm, and Python libraries should be available from the VFS (virtual filesystem).
  • Currently Python works with wasmtime, and should work with Enarx once it has VFS support.

Environment Setup


Install docker using the instructions here

CPython's Wasm build

Steps to build a CPython WebAssembly Build

git clone
  • Change directory to python-wasi using the command: cd python-wasi
  • Build the docker image using the command: docker build -f docker/Dockerfile -t wasi-build:latest docker
  • Now start the docker container and mount the current directory inside the docker container:
docker run -it --rm -v $(pwd):$(pwd) -w $(pwd) wasi-build:latest bash
  • To download the CPython source, dependencies and to build the CPython's Wasm build, run the command:
  • The build output is saved at opt/wasi-python/.

Python code

Python code to print the fibonacci sequence:

from functools import cache
import sys

def fib(n):
if n <= 0:
return 0
if n == 1:
return 1
return fib(n - 1) + fib(n - 2)

args = sys.argv[1:]

if len(args) == 0:
print("Please pass one or more numbers as arguments to the program")
for arg in args:
idx = int(arg)
print(f"Fibonacci sequence number at index {idx} is {fib(idx)}")

Access the Python codex repository for code samples, including the fibonacci example.

Run with Wasmtime

  • Change directory to the root python-wasi source directory.
  • Save the fibonacci code source at $HOME/
  • Run the python fibonacci code in wasmtime using the command
wasmtime run --mapdir=$(pwd)/opt::opt \
-- opt/wasi-python/bin/python3.wasm -c "$(cat $(pwd)/"