Add PyFrozenSet.add()

newsfragments/3156.added.md

@@ -0,0 +1 @@
+Add `PyFrozenSet.add()`, which is a valid operations at the C ABI level according to the [C API docs for sets](https://docs.python.org/3/c-api/set.html)

src/types/frozenset.rs

@@ -74,6 +74,22 @@ impl PyFrozenSet {
         }
     }
 
+    /// Adds an element to the set.
+    /// # Safety
+    ///
+    /// This is marked as unsafe because mutating a frozenset, which is conceptually immutable,
+    /// could be dangerous, even if it is allowed at the Python C ABI level.
+    /// You should only use this function if you are sure there are no other references to this frozenset.
+    pub unsafe fn add<K>(&self, key: K) -> PyResult<()>
+    where
+        K: ToPyObject,
+    {
+        err::error_on_minusone(
+            self.py(),
+            ffi::PySet_Add(self.as_ptr(), key.to_object(self.py()).as_ptr()),
+        )
+    }
+
     /// Returns an iterator of values in this frozen set.
     pub fn iter(&self) -> PyFrozenSetIterator<'_> {
         IntoIterator::into_iter(self)
@@ -222,6 +238,15 @@ mod tests {
         });
     }
 
+    #[test]
+    fn test_frozenset_add() {
+        Python::with_gil(|py| {
+            let set = PyFrozenSet::new(py, &[1, 2]).unwrap();
+            unsafe { set.add(1).unwrap() }; // Add a dupliated element
+            assert!(set.contains(1).unwrap());
+        });
+    }
+
     #[test]
     fn test_frozenset_iter() {
         Python::with_gil(|py| {

src/types/set.rs

@@ -46,14 +46,6 @@ impl PySet {
         unsafe { py.from_owned_ptr_or_err(ffi::PySet_New(ptr::null_mut())) }
     }
 
-    /// Removes all elements from the set.
-    #[inline]
-    pub fn clear(&self) {
-        unsafe {
-            ffi::PySet_Clear(self.as_ptr());
-        }
-    }
-
     /// Returns the number of items in the set.
     ///
     /// This is equivalent to the Python expression `len(self)`.
@@ -116,6 +108,14 @@ impl PySet {
         }
     }
 
+    /// Removes all elements from the set.
+    #[inline]
+    pub fn clear(&self) {
+        unsafe {
+            ffi::PySet_Clear(self.as_ptr());
+        }
+    }
+
     /// Returns an iterator of values in this set.
     ///
     /// # Panics