How to convert python bytes to Rust Vec<u8>?

[rdong8]

I thought this would work:

use pyo3::prelude::*;
use pyo3::types::PyBytes;

fn bytes_from_py(bytes: &Bound<'_, PyBytes>) -> Vec<u8> {
    bytes.as_bytes().to_vec()
}

#[pyfunction]
fn f(#[pyo3(from_py_with = "bytes_from_py")] bytes: Vec<u8>) {}

However, I end up with a non-primitve cast error:

non-primitive cast: `for<'a, 'b> fn(&'a pyo3::Bound<'b, pyo3::types::PyBytes>) -> Vec<u8> {bytes_from_py}` as `fn(&pyo3::Bound<'_, PyAny>) -> Result<Vec<u8>, PyErr>`
invalid cast

non-primitive cast: `fn bytes_from_py(&Bound<'_, PyBytes>) -> Vec<u8, Global>` as `fn(_) -> _`

[Icxolu]

The problem here is the signature of your bytes_from_py. To be usable with from_py_with it has to have to following signature: fn(&Bound<'_, PyAny>) -> PyResult<T>. Something like this should work:

fn bytes_from_py(bytes: &Bound<'_, PyAny>) -> PyResult<Vec<u8>> {
    Ok(bytes.downcast::<PyBytes>()?.as_bytes().to_vec())
}

[rdong8]

Thank you, this works! Might be an editor misconfiguration on my part, but I now have a warning like this:

warning[E0133]: call to unsafe function `pyo3::impl_::extract_argument::unwrap_required_argument` is unsafe and requires unsafe block
  --> src/lib.rs:36:63
   |
36 |         fn new(#[pyo3(from_py_with = "bytes_from_py")] bytes: Vec<u8>) -> PyResult<Self> {
   |                                                               ^^^^^^^ call to unsafe function
   |
   = note: for more information, see issue #71668 <https://github.com/rust-lang/rust/issues/71668>
   = note: consult the function's documentation for information on how to avoid undefined behavior
   = note: `#[warn(unsafe_op_in_unsafe_fn)]` on by default

It still compiles though. Is it a false positive?

[Icxolu]

That looks like a bug with PyO3's macro expansion. I recently fixed a bunch of these lint triggers in #4674, but this one must have slipped though. I'll open a PR to fix this, thanks for bringing it up.