[pydrake] Factor out make_shared_ptr_from_py_object for reuse (#22478)

bindings/pydrake/geometry/geometry_py_render.cc

@@ -88,18 +88,8 @@ class PyRenderEngine : public py::wrapper<RenderEngine> {
       }
       return copied;
     };
-    py::object py_engine = make_python_deepcopy();
-    // Convert the py_engine to a shared_ptr<RenderEngine> whose C++ lifetime
-    // keeps the python object alive.
-    RenderEngine* cpp_engine = py::cast<RenderEngine*>(py_engine);
-    DRAKE_DEMAND(cpp_engine != nullptr);
-    return std::shared_ptr<RenderEngine>(
-        /* stored pointer = */ cpp_engine,
-        /* deleter = */ [captured_py_engine = std::move(py_engine)](
-                            void*) mutable {
-          py::gil_scoped_acquire deleter_guard;
-          captured_py_engine = py::none();
-        });
+    py::object result = make_python_deepcopy();
+    return make_shared_ptr_from_py_object<RenderEngine>(result);
   }
 
   void DoRenderColorImage(ColorRenderCamera const& camera,

bindings/pydrake/pydrake_pybind.h

@@ -150,12 +150,30 @@ inline void ExecuteExtraPythonCode(py::module m, bool use_subdir = false) {
 /// anything -- it's managed object is null, so there is no reference counting.
 /// Calling get() on the result will return `raw`.
 template <typename T>
-auto make_unowned_shared_ptr_from_raw(T* raw) {
+std::shared_ptr<T> make_unowned_shared_ptr_from_raw(T* raw) {
   return std::shared_ptr<T>(
       /* managed object = */ std::shared_ptr<void>{},
       /* stored pointer = */ raw);
 }
 
+/// Given a Python object, returns a shared_ptr wrapper around it that keeps
+/// the Python object alive. If the py_object is None, returns nullptr. You
+/// must supply the expected C++ type to cast to as `T`.
+template <typename T>
+std::shared_ptr<T> make_shared_ptr_from_py_object(py::object py_object) {
+  if (py_object.is_none()) {
+    return {};
+  }
+  T* cpp_object = py::cast<T*>(py_object);
+  return std::shared_ptr<T>(
+      /* stored pointer = */ cpp_object,
+      /* deleter = */ [captured_py_object = std::move(py_object)](
+                          void*) mutable {
+        py::gil_scoped_acquire deleter_guard;
+        captured_py_object = py::none();
+      });
+}
+
 }  // namespace pydrake
 }  // namespace drake