Make CompoundRegion behave like Python iterable.

python/lsst/sphgeom/_compoundRegion.cc

@@ -67,11 +67,42 @@ std::unique_ptr<_CompoundRegion> _args_factory(const py::args& args) {
     return std::make_unique<_CompoundRegion>(std::move(operands));
 }
 
+// Iterator for CompoundRegion.
+class CompoundIterator {
+public:
+    CompoundIterator(CompoundRegion const& compound, size_t pos) : _compound(compound), _pos(pos) {}
+
+    Region const& operator*() const { return _compound.getOperand(_pos); }
+
+    CompoundIterator& operator++() {
+        ++ _pos;
+        return *this;
+    }
+
+    bool operator==(CompoundIterator const& other) const {
+        return &_compound == &other._compound and _pos == other._pos;
+    }
+
+private:
+    CompoundRegion const& _compound;
+    size_t _pos;
+};
+
 }  // namespace
 
 template <>
 void defineClass(py::class_<CompoundRegion, std::unique_ptr<CompoundRegion>, Region> &cls) {
     cls.def("nOperands", &CompoundRegion::nOperands);
+    cls.def("__len__", &CompoundRegion::nOperands);
+    cls.def(
+        "__iter__",
+        [](CompoundRegion const& region) {
+            return py::make_iterator(
+                CompoundIterator(region, 0U), CompoundIterator(region, region.nOperands())
+            );
+        },
+        py::return_value_policy::reference_internal  // Keeps region alive while iterator is in use.
+    );
     cls.def(
         "cloneOperand",
         [](CompoundRegion const &self, std::ptrdiff_t n) {

tests/test_CompoundRegion.py

@@ -122,6 +122,15 @@ def testOperands(self):
         """Test the cloneOperands accessor."""
         self.assertOperandsEqual(self.instance, self.operands)
 
+    def testIterator(self):
+        """Test Python iteration."""
+        self.assertEqual(len(self.instance), len(self.operands))
+        it = iter(self.instance)
+        self.assertEqual(next(it), self.operands[0])
+        self.assertEqual(next(it), self.operands[1])
+        with self.assertRaises(StopIteration):
+            next(it)
+
     def testCodec(self):
         """Test that encode and decode round-trip."""
         s = self.instance.encode()