Change PySet::new to accept a ExactSizeIterator

[samuelcolvin]

Somewhat related to #2673.

PyList::new has signature:

    pub fn new<T, U>(py: Python<'_>, elements: impl IntoIterator<Item = T, IntoIter = U>) -> &PyList
    where
        T: ToPyObject,
        U: ExactSizeIterator<Item = T>,
    {

But PySet::new is defined as

    pub fn new<'p, T: ToPyObject>(py: Python<'p>, elements: &[T]) -> PyResult<&'p PySet> {
        let list = elements.to_object(py);
        unsafe { py.from_owned_ptr_or_err(ffi::PySet_New(list.as_ptr())) }
    }

PySet::new elements parameter type should be relaxed to ExactSizeIterator<Item = T>, not a slice.

[adamreichold]

Shouldn't it actually take IntoIterator<IntoIter=U> where U: ExactSizeIterator, i.e. exactly the same signature as PyList::new?

[mejrs]

PyList requires ExactSizeIterator because it creates an empty list with a capacity (like vec::with_capacity). But PySet has no such api.

We can update PySet::new to just take an iterator and call PySet_Add in a loop.

[samuelcolvin]

Shouldn't it actually take IntoIterator<IntoIter=U> where U: ExactSizeIterator, i.e. exactly the same signature as PyList::new?

agreed.

PyList requires ExactSizeIterator because it creates an empty list with a capacity (like vec::with_capacity). But PySet has no such api.

We can update PySet::new to just take an iterator and call PySet_Add in a loop.

Not sure what would be fastest, but currently PySet::new is creating an intermediate PyList, hence the matching signature would make sense.

[davidhewitt]

My gut feeling is that using PySet_Add will be faster, because PySet_New is doing the iteration via Python dynamic protocol and also then we avoid the intermediate Python list allocation.

[samuelcolvin]

Then I guess there should be a from_list for the specific case where you already have a list?

[mejrs]

Then I guess there should be a from_list for the specific case where you already have a list?

If we let it take an IntoIterator, then users can just do PyList -> PyIterator -> PySet

[samuelcolvin]

Awesome, thank you.