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A complete and mature WebAssembly runtime for Python based on Wasmer.


  • Easy to use: The wasmer API mimics the standard WebAssembly API,
  • Fast: wasmer executes the WebAssembly modules as fast as possible, close to native speed,
  • Safe: All calls to WebAssembly will be fast, but more importantly, completely safe and sandboxed,
  • Modular: wasmer can compile the WebAssembly modules with different engines or compiler.

Documentation: browse the detailed API documentation full of examples.

Examples as tutorials: browse the examples/ directory, it's the best place for a complete introduction!

Quick Introduction

The wasmer package brings the required API to execute WebAssembly modules. In a nutshell, wasmer compiles the WebAssembly module into compiled code, and then executes it. wasmer is designed to work in various environments and platforms: From nano single-board computers to large and powerful servers, including more exotic ones. To address those requirements, Wasmer provides 2 engines and 3 compilers.

Succinctly, an engine is responsible to drive the compilation and the execution of a WebAssembly module. By extension, a headless engine can only execute a WebAssembly module, i.e. a module that has previously been compiled, or compiled, serialized and deserialized. By default, the wasmer package comes with 2 headless engines:

  1. wasmer.engine.JIT, the compiled machine code lives in memory,
  2. wasmer.engine.Native, the compiled machine code lives in a shared object file (.so, .dylib, or .dll), and is natively executed.

Because wasmer does not embed compilers in its package, engines are headless, i.e. they can't compile WebAssembly module; they can only execute them. Compilers live in their own standalone packages. Let's briefly introduce them:

Compiler package Description PyPi
wasmer_compiler_singlepass Super fast compilation times, slower execution times. Not prone to JIT-bombs. Ideal for blockchains On PyPi Downloads
wasmer_compiler_cranelift Fast compilation times, fast execution times. Ideal for development On PyPi Downloads
wasmer_compiler_llvm Slow compilation times, very fast execution times (close to native, sometimes faster). Ideal for Production On PyPi Downloads

We generally recommend wasmer_compiler_cranelift for development purposes and wasmer_compiler_llvm in production.

Learn more by reading the documentation of the wasmer.engine submodule.


To install the wasmer Python package, and let's say the wasmer_compiler_cranelift compiler, just run those commands in your shell:

$ pip install wasmer==1.0.0
$ pip install wasmer_compiler_cranelift==1.0.0

And you're ready to get fun!


We highly recommend to read the examples/ directory, which contains a sequence of examples/tutorials. It's the best place to learn by reading examples.

But for the most eager of you, and we know you're numerous you mischievous, there is a quick toy program in examples/appendices/, written in Rust:

pub extern fn sum(x: i32, y: i32) -> i32 {
    x + y

After compilation to WebAssembly, the examples/appendices/simple.wasm binary file is generated. (Download it).

Then, we can execute it in Python:

from wasmer import engine, Store, Module, Instance
from wasmer_compiler_cranelift import Compiler

# Let's define the store, that holds the engine, that holds the compiler.
store = Store(engine.JIT(Compiler))

# Let's compile the module to be able to execute it!
module = Module(store, open('simple.wasm', 'rb').read())

# Now the module is compiled, we can instantiate it.
instance = Instance(module)

# Call the exported `sum` function.
result = instance.exports.sum(5, 37)

print(result) # 42!

And then, finally, enjoy by running:

$ python examples/appendices/


The Python extension is written in Rust, with pyo3 and maturin.

First, you need to install Rust and Python. We will not make you the affront to explain to you how to install Python (if you really need, check pyenv). For Rust though, we advise to use rustup, then:

$ rustup install stable

To set up your environment, you'll need just, and then, install the prelude of this project:

$ cargo install just
$ just --list # to learn about all the available recipes
$ just prelude

It will install pyo3 and maturin for Python and for Rust. It will also install virtualenv.

Then, simply run:

$ source .env/bin/activate
$ just build api
$ just build compiler-cranelift
$ python examples/appendices/

Supported platforms

We try to provide wheels for as many platforms and architectures as possible. For the moment, here are the supported platforms and architectures:

Platform Architecture Triple Packages
Linux amd64 x86_64-unknown-linux-gnu wasmer
aarch64 aarch64-unknown-linux-gnu wasmer
wasmer_compiler_singlepass 1
Darwin amd64 x86_64-apple-darwin wasmer
Windows amd64 x86_64-pc-windows-msvc wasmer
wasmer_compiler_llvm 2


  • 1 wasmer_compiler_singlepass does not support aarch64 for the moment
  • 2 wasmer_compiler_llvm is not packaging properly on Windows for the moment

Wheels are all built for the following Python versions:

  • Python 3.6,
  • Python 3.7,
  • Python 3.8.
  • Python 3.9.
Learn about the “fallback” py3-none-any wheel


A special wasmer-$(version)-py3-none-any wheel is built as a fallback. The wasmer libray will be installable, but it will raise an ImportError exception saying that “Wasmer is not available on this system”.

This wheel will be installed if none matches before (learn more by reading the PEP 425, Compatibility Tags for Built Distributions).


Build all the packages and run the tests:

$ just build-all
$ just test

What is WebAssembly?

Quoting the WebAssembly site:

WebAssembly (abbreviated Wasm) is a binary instruction format for a stack-based virtual machine. Wasm is designed as a portable target for compilation of high-level languages like C/C++/Rust, enabling deployment on the web for client and server applications.

About speed:

WebAssembly aims to execute at native speed by taking advantage of common hardware capabilities available on a wide range of platforms.

About safety:

WebAssembly describes a memory-safe, sandboxed execution environment […].


The entire project is under the MIT License. Please read the LICENSE file.