Added `size_hint` impls for `{PyDict,PyList,PySet,PyTuple}Iterator`s

CHANGELOG.md

@@ -115,6 +115,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 - Add `serde` feature which provides implementations of `Serialize` and `Deserialize` for `Py<T>`. [#1366](https://github.com/PyO3/pyo3/pull/1366)
 - Add FFI definition `_PyCFunctionFastWithKeywords` on Python 3.7 and up. [#1384](https://github.com/PyO3/pyo3/pull/1384)
 - Add `PyDateTime::new_with_fold()` method. [#1398](https://github.com/PyO3/pyo3/pull/1398)
+- Add `size_hint` impls for `{PyDict,PyList,PySet,PyTuple}Iterator`s. [#1699](https://github.com/PyO3/pyo3/pull/1699)
 
 ### Changed
 

src/types/dict.rs

@@ -204,6 +204,15 @@ impl<'py> Iterator for PyDictIterator<'py> {
             }
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.dict.len().unwrap_or_default();
+        (
+            len.saturating_sub(self.pos as usize),
+            Some(len.saturating_sub(self.pos as usize)),
+        )
+    }
 }
 
 impl<'a> std::iter::IntoIterator for &'a PyDict {
@@ -698,6 +707,28 @@ mod test {
         assert_eq!(32 + 42 + 123, value_sum);
     }
 
+    #[test]
+    fn test_iter_size_hint() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let mut v = HashMap::new();
+        v.insert(7, 32);
+        v.insert(8, 42);
+        v.insert(9, 123);
+        let ob = v.to_object(py);
+        let dict = <PyDict as PyTryFrom>::try_from(ob.as_ref(py)).unwrap();
+
+        let mut iter = dict.iter();
+        assert_eq!(iter.size_hint(), (v.len(), Some(v.len())));
+        iter.next();
+        assert_eq!(iter.size_hint(), (v.len() - 1, Some(v.len() - 1)));
+
+        // Exhust iterator.
+        while let Some(_) = iter.next() {}
+
+        assert_eq!(iter.size_hint(), (0, Some(0)));
+    }
+
     #[test]
     fn test_into_iter() {
         let gil = Python::acquire_gil();

src/types/list.rs

@@ -158,6 +158,16 @@ impl<'a> Iterator for PyListIterator<'a> {
             None
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.list.len();
+
+        (
+            len.saturating_sub(self.index as usize),
+            Some(len.saturating_sub(self.index as usize)),
+        )
+    }
 }
 
 impl<'a> std::iter::IntoIterator for &'a PyList {
@@ -342,6 +352,25 @@ mod test {
         assert_eq!(idx, v.len());
     }
 
+    #[test]
+    fn test_iter_size_hint() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+        let v = vec![2, 3, 5, 7];
+        let ob = v.to_object(py);
+        let list = <PyList as PyTryFrom>::try_from(ob.as_ref(py)).unwrap();
+
+        let mut iter = list.iter();
+        assert_eq!(iter.size_hint(), (v.len(), Some(v.len())));
+        iter.next();
+        assert_eq!(iter.size_hint(), (v.len() - 1, Some(v.len() - 1)));
+
+        // Exhust iterator.
+        while let Some(_) = iter.next() {}
+
+        assert_eq!(iter.size_hint(), (0, Some(0)));
+    }
+
     #[test]
     fn test_into_iter() {
         let gil = Python::acquire_gil();

src/types/set.rs

@@ -172,6 +172,15 @@ impl<'py> Iterator for PySetIterator<'py> {
             }
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.set.len().unwrap_or_default();
+        (
+            len.saturating_sub(self.pos as usize),
+            Some(len.saturating_sub(self.pos as usize)),
+        )
+    }
 }
 
 impl<'a> std::iter::IntoIterator for &'a PySet {
@@ -505,6 +514,24 @@ mod test {
         }
     }
 
+    #[test]
+    fn test_set_iter_size_hint() {
+        let gil = Python::acquire_gil();
+        let py = gil.python();
+
+        let set = PySet::new(py, &[1]).unwrap();
+
+        let mut iter = set.iter();
+
+        if cfg!(Py_LIMITED_API) {
+            assert_eq!(iter.size_hint(), (0, None));
+        } else {
+            assert_eq!(iter.size_hint(), (1, Some(1)));
+            iter.next();
+            assert_eq!(iter.size_hint(), (0, Some(0)));
+        }
+    }
+
     #[test]
     fn test_frozenset_new_and_len() {
         let gil = Python::acquire_gil();

src/types/tuple.rs

@@ -131,6 +131,14 @@ impl<'a> Iterator for PyTupleIterator<'a> {
             None
         }
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (
+            self.length.saturating_sub(self.index as usize),
+            Some(self.length.saturating_sub(self.index as usize)),
+        )
+    }
 }
 
 impl<'a> ExactSizeIterator for PyTupleIterator<'a> {
@@ -346,9 +354,17 @@ mod test {
         let tuple = <PyTuple as PyTryFrom>::try_from(ob.as_ref(py)).unwrap();
         assert_eq!(3, tuple.len());
         let mut iter = tuple.iter();
+
+        assert_eq!(iter.size_hint(), (3, Some(3)));
+
         assert_eq!(1, iter.next().unwrap().extract().unwrap());
+        assert_eq!(iter.size_hint(), (2, Some(2)));
+
         assert_eq!(2, iter.next().unwrap().extract().unwrap());
+        assert_eq!(iter.size_hint(), (1, Some(1)));
+
         assert_eq!(3, iter.next().unwrap().extract().unwrap());
+        assert_eq!(iter.size_hint(), (0, Some(0)));
     }
 
     #[test]