How to get a list of strings from Python to Rust

[IdoPort]

Hi all!
I am trying to write my first pyo3 module in Rust and I am having some difficulties.
My first use case is text analysis function that takes a string and a list of indicators and looks for them inside the string.
Here is my attempt:
#[pyfunction] fn analyze_string(a: &str, b: Vec<&str>) -> PyResult<bool> { Ok(false) }

From what I understand in the docs it should work, however when I try to compile it I get the following:

error[E0432]: unresolved import 'vec_of_str'                                                                                                                                                               
  --> src\lib.rs:12:20
   |
12 |     m.add_function(wrap_pyfunction!(vec_of_str, m)?)?;
   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no external crate 'vec_of_str'
   |
   = note: this error originates in the macro 'wrap_pyfunction' (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound 'Vec<&str>: PyFunctionArgument<'_, '_>' is not satisfied                                                                                                                     
   --> src\lib.rs:5:31
    |
5   | fn analyze_string(a: &str, b: Vec<&str>) -> PyResult<bool> {
    |                               ^^^ the trait 'FromPyObject<'_>' is not implemented for 'Vec<&str>', which is required by 'Vec<&str>: PyFunctionArgument<'_, '_>'
    |
    = help: the trait 'FromPyObject<'_>' is implemented for 'Vec<T>'
    = note: required for 'Vec<&str>' to implement 'FromPyObjectBound<'_, '_>'
    = note: required for 'Vec<&str>' to implement 'PyFunctionArgument<'_, '_>'
note: required by a bound in 'extract_argument'
   --> C:\Users\Ido\.cargo\registry\src\index.crates.io-6f17d22bba15001f\pyo3-0.23.3\src\impl_\extract_argument.rs:119:8
    |
113 | pub fn extract_argument<'a, 'py, T>(
    |        ---------------- required by a bound in this function
...
119 |     T: PyFunctionArgument<'a, 'py>,
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in 'extract_argument'

I am not sure how I should fix that, and since this seems like a basic primitive I thought it must have a good reason not to work out of the box.

All documentation I find either tell me it should work or are from over 3 years which I suspect are not relevant yet.
Could someone help me?

[LilyFoote]

Hi @IdoPort!

Can you share the full code of your src\lib.rs file? That would make it easier to help you.

That said, I was able to modify your analyze_string function to:

#[pyfunction]
fn analyze_string(a: &str, b: Vec<String>) -> PyResult<bool> {
    Ok(false)
}

and it now compiles. You might find the table of conversions between Python and Rust types useful. In this case, I'm not certain why we don't have an implementation of FromPyObject for Vec<&str> - perhaps there's some technical limitation that makes it difficult.

Depending on your use case, you might also be able to use either:

use pyo3::types::PyString;

#[pyfunction]
fn analyze_string(a: &str, b: Vec<Bound<'_, PyString>>) -> PyResult<bool> {
    Ok(false)
}

or

use pyo3::pybacked::PyBackedStr;

#[pyfunction]
fn analyze_string(a: &str, b: Vec<PyBackedStr>) -> PyResult<bool> {
    Ok(false)
}

but I'd try the Vec<String> option first.

[IdoPort]

Thank you very much!
I have tried it before and I thought it made me do many unnecessary copies but I was wrong.

If someone has a good explanation to why Vec<&str> is not supported it would be nice to know.

[davidhewitt]

See #3997 (comment)