Allow non-ExactSizeIterator in PyList::new

newsfragments/5830.changed.md

@@ -0,0 +1 @@
+Allow non-`ExactSizeIterator` in `PyList::new`

src/types/list.rs

@@ -36,43 +36,6 @@ impl crate::impl_::pyclass::PyClassBaseType for PyList {
         crate::impl_::pycell::PyVariableClassObject<T>;
 }
 
-#[inline]
-#[track_caller]
-fn try_new_from_iter<'py>(
-    py: Python<'py>,
-    mut elements: impl ExactSizeIterator<Item = PyResult<Bound<'py, PyAny>>>,
-) -> PyResult<Bound<'py, PyList>> {
-    unsafe {
-        // PyList_New checks for overflow but has a bad error message, so we check ourselves
-        let len: Py_ssize_t = elements
-            .len()
-            .try_into()
-            .expect("out of range integral type conversion attempted on `elements.len()`");
-
-        let ptr = ffi::PyList_New(len);
-
-        // We create the `Bound` pointer here for two reasons:
-        // - panics if the ptr is null
-        // - its Drop cleans up the list if user code or the asserts panic.
-        let list = ptr.assume_owned(py).cast_into_unchecked();
-
-        let count = (&mut elements)
-            .take(len as usize)
-            .try_fold(0, |count, item| {
-                #[cfg(not(Py_LIMITED_API))]
-                ffi::PyList_SET_ITEM(ptr, count, item?.into_ptr());
-                #[cfg(Py_LIMITED_API)]
-                ffi::PyList_SetItem(ptr, count, item?.into_ptr());
-                Ok::<_, PyErr>(count + 1)
-            })?;
-
-        assert!(elements.next().is_none(), "Attempted to create PyList but `elements` was larger than reported by its `ExactSizeIterator` implementation.");
-        assert_eq!(len, count, "Attempted to create PyList but `elements` was smaller than reported by its `ExactSizeIterator` implementation.");
-
-        Ok(list)
-    }
-}
-
 impl PyList {
     /// Constructs a new list with the given elements.
     ///
@@ -101,16 +64,38 @@ impl PyList {
     /// All standard library structures implement this trait correctly, if they do, so calling this
     /// function with (for example) [`Vec`]`<T>` or `&[T]` will always succeed.
     #[track_caller]
-    pub fn new<'py, T, U>(
+    pub fn new<'py, T>(
         py: Python<'py>,
-        elements: impl IntoIterator<Item = T, IntoIter = U>,
+        elements: impl IntoIterator<Item = T>,
     ) -> PyResult<Bound<'py, PyList>>
     where
         T: IntoPyObject<'py>,
-        U: ExactSizeIterator<Item = T>,
     {
-        let iter = elements.into_iter().map(|e| e.into_bound_py_any(py));
-        try_new_from_iter(py, iter)
+        let mut elements = elements.into_iter().map(|e| e.into_bound_py_any(py));
+        let (min_len, _) = elements.size_hint();
+
+        // PyList_New checks for overflow but has a bad error message, so we check ourselves
+        let len: Py_ssize_t = min_len
+            .try_into()
+            .expect("out of range integral type conversion attempted on `elements.len()`");
+
+        let list = unsafe { ffi::PyList_New(len).assume_owned(py).cast_into_unchecked() };
+
+        let count = (&mut elements)
+            .take(len as usize)
+            .try_fold(0, |count, item| unsafe {
+                #[cfg(not(Py_LIMITED_API))]
+                ffi::PyList_SET_ITEM(list.as_ptr(), count, item?.into_ptr());
+                #[cfg(Py_LIMITED_API)]
+                ffi::PyList_SetItem(list.as_ptr(), count, item?.into_ptr());
+                Ok::<_, PyErr>(count + 1)
+            })?;
+
+        assert_eq!(len, count, "Attempted to create PyList but `elements` was smaller than reported by its `size_hint` implementation.");
+
+        elements.try_for_each(|item| list.append(item?))?;
+
+        Ok(list)
     }
 
     /// Constructs a new empty list.
@@ -1516,6 +1501,10 @@ mod tests {
         fn next(&mut self) -> Option<Self::Item> {
             self.0.next()
         }
+
+        fn size_hint(&self) -> (usize, Option<usize>) {
+            (self.1, Some(self.1))
+        }
     }
 
     impl ExactSizeIterator for FaultyIter {
@@ -1526,18 +1515,7 @@ mod tests {
 
     #[test]
     #[should_panic(
-        expected = "Attempted to create PyList but `elements` was larger than reported by its `ExactSizeIterator` implementation."
-    )]
-    fn too_long_iterator() {
-        Python::attach(|py| {
-            let iter = FaultyIter(0..usize::MAX, 73);
-            let _list = PyList::new(py, iter).unwrap();
-        })
-    }
-
-    #[test]
-    #[should_panic(
-        expected = "Attempted to create PyList but `elements` was smaller than reported by its `ExactSizeIterator` implementation."
+        expected = "Attempted to create PyList but `elements` was smaller than reported by its `size_hint` implementation."
     )]
     fn too_short_iterator() {
         Python::attach(|py| {