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ports/webassembly/proxy_c.c: Allow python bytes() to be proxied to/from JS. #17044

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ports/webassembly/proxy_c.c: Allow python bytes() to be proxied to/from JS. #17044

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@bwhitman bwhitman commented Mar 30, 2025 •
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Summary

This allows python bytes() objects to be passed to JS. Example:

 function set_audio_samples(samples) {
     var res_ptr = amy_module._malloc(2 * 256 * 2); // 2 channels, 256 frames, int16s
     amy_module.HEAPU8.set(samples, res_ptr);
     amy_module.ccall('amy_set_input_buffer', null, ["number"], [res_ptr]);
     amy_module._free(res_ptr);
 }
...
 await mp.registerJsModule('amy_set_input_buffer', set_audio_samples);

In MP:

buffer = bytes(1024)
# do something to fill in the bytes...
amy_set_input_buffer(buffer)

It also works the other way (JS returning bytes() to MP):

 function get_audio_samples() {
     var res_ptr = amy_module._malloc(2 * 256 * 2); // 2 channels, 256 frames, int16s
     amy_module.ccall('amy_get_input_buffer', null, ["number"], [res_ptr]);
     var view = amy_module.HEAPU8.subarray(res_ptr , res_ptr + 1024);
     amy_module._free(res_ptr);
     return view
 }
 await mp.registerJsModule('amy_get_input_buffer', get_audio_samples);
>>> b = amy_get_input_buffer()
>>> type(t)
<class 'bytes'>

Previously, we would have to manually construct a js.UInt8Array.new(1024) and fill it in one by one.

Testing

Tested on webassembly port.

@bwhitman bwhitman changed the title ports/webassembly/proxy_c.c: allow python bytes() to be proxied to JS ports/webassembly/proxy_c.c: Allow python bytes() to be proxied to/from JS. Mar 30, 2025
@dpgeorge
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Thanks for the contribution, this is definitely a useful feature and makes it much more efficient to pass bytes between Python and JS.

But, because Uint8Array is a mutable object, it no longer allows you to share such an object between Python and JS (instead this PR would make a copy when passing it across the divide).

At the moment you can do something like this:

const mp = await (await import(process.argv[2])).loadMicroPython();

globalThis.array = new Uint8Array(4);
globalThis.array[0] = 1;
console.log(globalThis.array);

mp.runPython(`
import js
print(js.array[0])  # access JS Uint8Array
js.array[0] = 2  # modify JS Uint8Array
`);

console.log(globalThis.array);

Running this, it outputs:

$ node test.mjs `pwd`/build-pyscript/micropython.mjs
Uint8Array(4) [ 1, 0, 0, 0 ]
1
Uint8Array(4) [ 2, 0, 0, 0 ]

I don't think such sharing of mutable Uint8Array will be possible with this PR.

Maybe instead it could be part of jsffi.to_js() where the user explicitly converts the value?

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