How to call the wasm library generated by rust using python wasmtime

This article describes a convenient way to access the rust library using python wasmtime in extremely concise steps that can be used in a production environment.

Install rust target wasm32-wasi

Use the following command to install wasm32-wasi for conveniently compiling rust to wasm bytecode.

rustup target add wasm32-wasi

Preparation of rust libraries

Creating a rust library with cargo

cd /mnt/d/workspace/rust
cargo new --lib wasmlib

Edit file

cd wasmlib
echo "[lib]" >> 
echo "crate-type = ['cdylib']" >>

Edit rust source file
compilersrc/, modify the document to read as follows

#[no_mangle]
pub extern "C" fn add(a: i32, b: i32) -> i32 {
    a + b
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        let result = add(2, 2);
        assert_eq!(result, 4);
    }
}

Compile rust library to wasm bytecode

Compile rust library to wasm bytecode

cargo build --release --target wasm32-wasi

Viewing the generated wasm file

cd target/wasm32-wasi/release
ls

The result is as follows, which is the wasm bytecode file:

build  deps  examples  incremental

Installing the python wasmtime library

pip3 install wasmtime

Test calling the rust library using python wasmtime (wasm bytecode)

hzw@hzwwin:/mnt/d/workspace/rust/wasmlib/target/wasm32-wasi/release$ python3
Python 3.10.4 (main, Apr  2 2022, 09:04:19) [GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import 
>>> import wasmlib # Without the .wasm extension
>>> dir(wasmlib)
['__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'add', 'memory']
>>> (2, 2)
4
>>> (2, 4)
6
>>> ("a", "b")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/hzw/.local/lib/python3.10/site-packages/wasmtime/", line 89, in <lambda>
    item = lambda *args,func=func: func(store, *args)  # noqa
  File "/home/hzw/.local/lib/python3.10/site-packages/wasmtime/_func.py", line 83, in __call__
    param_vals = [Val._convert(ty, params[i]) for i, ty in enumerate(param_tys)]
  File "/home/hzw/.local/lib/python3.10/site-packages/wasmtime/_func.py", line 83, in <listcomp>
    param_vals = [Val._convert(ty, params[i]) for i, ty in enumerate(param_tys)]
  File "/home/hzw/.local/lib/python3.10/site-packages/wasmtime/_value.py", line 154, in _convert
    raise TypeError("don't know how to convert %r to %s" % (val, ty))
TypeError: don't know how to convert 'a' to i32

Reference Links

/project/wasmtime/

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