Passing a default, variable-length list of strings in a function signature

[jdaymude]

I have a function of the form:

#[pyfunction]
pub fn example(a: Vec<String>) -> PyResult<bool> {
    // Do things.
    Ok(true)
}

The variable a is meant to be a variable-length list of Python strings. This compiles and works great, but if I try to provide a function signature with defaults...

#[pyfunction]
#[pyo3(signature = (a=["x", "y", "z"]))]
pub fn example(a: Vec<String>) -> PyResult<bool> {
    // Do things.
    Ok(true)
}

...then I get the error:

error[E0277]: the trait bound `[&str; 3]: PyFunctionArgument<'_, '_, false>` is not satisfied
   --> src/python.rs:xxx:xxx
    |
xxx |     a: Vec<String>,
    |        ^^^ the trait `pyo3::FromPyObject<'_>` is not implemented for `[&str; 3]`
    |
    = help: the trait `pyo3::FromPyObject<'_>` is implemented for `[T; N]`
    = note: required for `[&str; 3]` to implement `FromPyObjectBound<'_, '_>`
    = note: required for `[&str; 3]` to implement `PyFunctionArgument<'_, '_, false>`
note: required by a bound in `extract_argument_with_default`
   --> /path/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pyo3-0.24.2/src/impl_/extract_argument.rs:161:8
    |
154 | pub fn extract_argument_with_default<'a, 'py, T, const IS_OPTION: bool>(
    |        ----------------------------- required by a bound in this function
...
161 |     T: PyFunctionArgument<'a, 'py, IS_OPTION>,
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `extract_argument_with_default`

error[E0308]: `?` operator has incompatible types
   --> src/python.rs:xxx:xxx
    |
xxx |     a: Vec<String>,
    |        ^^^ expected `Vec<String>`, found `[&str; 3]`
    |
    = note: `?` operator cannot convert from `[&str; 3]` to `Vec<std::string::String>`
    = note: expected struct `Vec<std::string::String>`
                found array `[&str; 3]`

The pyo3 user guide states (emphasis added):

• arg=Value: arguments with default value. If the arg argument is defined after var arguments, it is treated as a keyword-only argument. Note that Value has to be valid rust code, PyO3 just inserts it into the generated code unmodified.

So I also tried:

#[pyfunction]
#[pyo3(signature = (a=vec!["x", "y", "z"]))]
pub fn example(a: Vec<String>) -> PyResult<bool> {
    // Do things.
    Ok(true)
}

But this yields a similar error:

error[E0308]: `?` operator has incompatible types
   --> src/python.rs:xxx:xxx
    |
xxx |     a: Vec<String>,
    |        ^^^ expected `Vec<String>`, found `Vec<&str>`
    |
    = note: `?` operator cannot convert from `Vec<&str>` to `Vec<std::string::String>`
    = note: expected struct `Vec<std::string::String>`
               found struct `Vec<&str>`

I have read #4830 and reviewed the table of argument types, but the suggested solutions didn't help (just variations of the above error about being unable to convert Vec<&str> to vectors of String, PyString, or PyBackedStr). I could go without setting a default, but would really like to get that working. Any suggestions?

[jdaymude]

Ack, okay, I am incredibly dumb. This is not a pyo3 issue, this is an "I am bad at confusing Python and Rust" issue. The correct way to do this is:

#[pyfunction]
#[pyo3(signature = (a=vec!["x".to_string(), "y".to_string(), "z".to_string()]))]
pub fn example(a: Vec<String>) -> PyResult<bool> {
    // Do things.
    Ok(true)
}

For good measure, it may be useful to also override the __text_signature__ to hide all this Rust-specific stuff.