Implement IntoIterator for PySet and PyFrozenSet

[davidhewitt]

Closes #715

[kngwyu]

Could you please rewrite the implementation using _PySet_NextEntry?

1. Add this declaration in ffi::setobject.rs

#[cfg(not(Py_LIMITED_API))]
extern "C" {
    #[cfg_attr(PyPy, link_name = "_PySet_NextEntry")]
    pub fn _PySet_NextEntry(
        set: *mut PyObject,
        pos: *mut Py_ssize_t,
        key: *mut *mut PyObject,
        hash: *mut super::Py_hash_t,
    ) -> c_int;
}

2. Then implementation comes like

#[cfg(not(Py_LIMITED_API))]
pub struct PySetIterator<'py> {
    set: &'py super::PyAny,
    pos: isize,
}

#[cfg(not(Py_LIMITED_API))]
impl<'py> Iterator for PySetIterator<'py> {
    type Item = &'py super::PyAny;

    #[inline]
    fn next(&mut self) -> Option<Self::Item> {
        unsafe {
            let mut key: *mut ffi::PyObject = std::ptr::null_mut();
            let mut hash: ffi::Py_hash_t = 0;
            if ffi::_PySet_NextEntry(self.set.as_ptr(), &mut self.pos, &mut key, &mut hash) != 0 {
                Some(self.set.py().from_borrowed_ptr(key))
            } else {
                None
            }
        }
    }
}

#[cfg(not(Py_LIMITED_API))]
impl<'a> std::iter::IntoIterator for &'a PySet {
    type Item = &'a PyAny;
    type IntoIter = PySetIterator<'a>;

    fn into_iter(self) -> Self::IntoIter {
        PySetIterator {
            set: self.as_ref(),
            pos: 0,
        }
    }
}

[davidhewitt]

👍 Thanks - I missed _PySet_Next because I didn't find it at https://docs.python.org/3/c-api/set.html!

[kngwyu]

Thank you!

[samuelcolvin]

Thanks for fixing this.

My only concern is that PySet still doesn't implement iter() as an explicit method (i'm not sure how that's being implemented, but I guess it must be as it's working in tests) so it's not included in the documentation.

For a list (ignoring the comment that I guess is left over from copying the method from tuples) we have:

pyo3/src/types/list.rs

Lines 116 to 122 in b3ca27d

	/// Returns an iterator over the tuple items.
	pub fn iter(&self) -> PyListIterator {
	PyListIterator {
	list: self,
	index: 0,
	}
	}

Would it help to add iter() to sets? It's trivial enough that even I can create a PR.

[davidhewitt]

I left .iter() off because I didn't see a need for it, given IntoIterator is now implemented and that'll be in the docs.

If you still would like to have iter(), please do open a PR. Note you'd need to wrap any iter() method you add in #[cfg(not(Py_LIMITED_API))], like the other definitions we just merged.

[samuelcolvin]

okay, I'll leave it.

[kngwyu]

@samuelcolvin
I implemented it in #717, for the compatibility with PyDict.