How to use Vec<&str> as pyo3 struct field?

[tushushu]

It seems that the lifetime is not supported in pyo3, so what should I do if I need declare the struct field as Vec<&str> type?
Vec<String> is not used here because:

1. There would be a lot of duplicates in the vector;
2. The elements of the vector are not mutable.
   Thanks very much~

[birkenfeld]

What you could normally use in Rust for such cases is Vec<Rc<str>> which avoids the duplicates. I don't think this can be directly be exposed to Python though.

What about Vec<Py<PyString>>? If you have to expose the strings to Python, you'll have to convert them to PyString at some point anyway, and Py is like Rc in that you can clone them by just making a new reference. Python strings are also immutable.

[tushushu]

Thanks @birkenfeld , I am trying to write a small Python project to process array-like object more efficiently. So I prefer to use native rust string type here.

[mejrs]

Can you give an example of what you're trying to do?

If you are trying to have structs like this...

struct Foo<'a> {
    things: Vec<&'a str>
}

...then someone else will have to own whatever those &str's are pointing to. As such these structs don't work very well because they'll be bound to something else. You almost always want structs to own their fields.

That's true in general and not specific to pyo3. But when you add python into the mix and give this struct to it, it can keep it alive forever. As such, whatever is backing the references has to remain alive forever. This is why the pyclass macro forbids lifetime parameters.

[tushushu]

Currently the implementation is as below, I am using RefCell<Vec<String>> instead of RefCell<Vec<&str>> to avoid the lifetime issue. Full implementation

When handling things like 'reques_id' or 'uuid' it's fine because there are little duplicate values.

But when processing things like 'gender' or 'education background', then it's a waste of memory. For there are only 3 or 4 unique elements, we'd better not creating so many String objects on the heap.

I am also thinking of having a HashMap in the struct to keep the unique strings as keys and number of this key in the vector as values, and having a Vec to point to the HashMap. But maybe this is too tricky, and there may be a simple way to solve this issue?

By the way, I am new to Rust. So if there is anything I said is inaccurate please correct me, thanks very much.

#[pyclass]
pub struct StringList {
    _values: RefCell<Vec<String>>,
}

#[pymethods]
impl StringList {
    // Arrange the following methods in alphabetical order.

    #[new]
    pub fn new(vec: Vec<String>) -> Self {
        List::_new(vec)
    }

    pub fn append(&self, elem: String) {
        List::append(self, elem)
    }

    pub fn as_bool(&self) -> BooleanList {
        AsBooleanList::as_bool(self)
    }

    ......

    pub fn to_list(&self) -> Vec<String> {
        List::to_list(self)
    }

    pub fn union_all(&self, other: &Self) -> Self {
        List::union_all(self, other)
    }

    pub fn unique(&self) -> Self {
        NonFloatList::unique(self)
    }
}

[mejrs]

If you want to share the string, you probably want a Rc<str>.

things like 'gender' or 'education background'

Try using enums for these rather than strings, if you can.

Also - unless I'm missing something I'm not sure if you need RefCell, since you're not sharing it.

[tushushu]

If you want to share the string, you probably want a Rc<str>.

things like 'gender' or 'education background'

Try using enums for these rather than strings, if you can.

Also - unless I'm missing something I'm not sure if you need RefCell, since you're not sharing it.

Yeah, let me try Rc<str> and enums. I am using RefCell here to get mutability because the struct field Vec<T> is immutable.
Thanks very much for helping me with this. @mejrs @birkenfeld