Python Tooling

Building a Component with componentize-py

componentize-py is a tool that converts a Python application to a WebAssembly component.

First, install Python 3.10 or later and pip if you don't already have them. Then, install componentize-py:

pip install componentize-py

Next, create or download the WIT world you would like to target. For this example we will use an example world with an add function:

package example:component;

world example {
    export add: func(x: s32, y: s32) -> s32;

If you want to generate bindings produced for the WIT world (for an IDE or typechecker), you can generate them using the bindings subcommand. Specify the path to the WIT interface with the world you are targeting:

$ componentize-py --wit-path /path/to/examples/example-host/add.wit --world example bindings .

Note: you do not need to generate the bindings in order to componentize in the next step. componentize will generate bindings on-the-fly and bundle them into the produced component.

You can see that bindings were created in an example package which contains an Example protocol with an add method that we can implement:

$ cat<<EOT >>
import example

class Example(example.Example):
    def add(self, x: int, y: int) -> int:
        return x + y

We now can compile our application to a Wasm component using the componentize subcommand:

$ componentize-py --wit-path /path/to/examples/example-host/add.wit --world example componentize app -o add.wasm
Component built successfully

To test the component, run it using the Rust add host:

$ cd component-model/examples/add-host
$ cargo run --release -- 1 2 ../path/to/add.wasm
1 + 2 = 3

See componentize-py's examples to try out build HTTP, CLI, and TCP components from Python applications.

Building a Component that Exports an Interface

The sample add.wit file exports a function. However, to use your component from another component, it must export an interface. That being said, you rarely find WIT that does not contain an interface. (Most WITs you'll see in the wild do use interfaces; we've been simplifying by exporting a function.) Let's expand our example world to export an interface rather than directly export the function.

// add-interface.wit
package example:component;

interface add {
    add: func(a: u32, b: u32) -> u32;

world example {
    export add;

If you peek at the bindings, you'll notice that we now implement a class for the add interface rather than for the example world. This is a consistent pattern. As you export more interfaces from your world, you implement more classes. Our add example gets the slight update of:

import example

class Add(example.Example):
    def add(self, a: int, b: int) -> int:
        return a + b

Once again, compile an application to a Wasm component using the componentize subcommand:

$ componentize-py --wit-path add-interface.wit --world example componentize app -o add.wasm
Component built successfully

Running components from Python Applications

Wasm components can also be invoked from Python applications. This walks through using tooling to call the app.wasm component from the examples.

Note: wasmtime-py does not currently support running components build with componentize-py. This is because wasmtime-py does not yet support resources, which components built with componentize-py always use, since componentize-py unconditionally imports most of the wasi:cli world.

First, install Python 3.11 or later and pip if you don't already have them. Then, install wasmtime-py:

$ pip install wasmtime

First, generate the bindings to be able to call the component from a Python host application.

# Get an add component that does not import the WASI CLI world
$ wget
$ python3 -m wasmtime.bindgen add.wasm --out-dir add

The generated package add has all of the requisite exports/imports for the component and is annotated with types to assist with type-checking and self-documentation as much as possible. Inside the package is a Root class with an add function that calls the component's exported add function. We can now write a Python program that calls add:

from add import Root
from wasmtime import Store

def main():
    store = Store()
    component = Root(store)
    print("1 + 2 = ", component.add(store, 1, 2))

if __name__ == '__main__':

Run the Python host program:

$ python3
1 + 2 = 3