From c560eb6ef2e3e7542815f27a69d9f4462587a6f8 Mon Sep 17 00:00:00 2001
From: Jeremy Nimmer <jeremy.nimmer@tri.global>
Date: Tue, 31 May 2022 08:19:01 -0700
Subject: [PATCH] [pydrake] Add bindings for SharedPointerSystem (#17264)

* [pydrake] Add bindings for SharedPointerSystem
---
 bindings/pydrake/systems/primitives_py.cc     | 30 +++++++++++++++++++
 .../pydrake/systems/test/primitives_test.py   | 22 ++++++++++++++
 2 files changed, 52 insertions(+)

diff --git a/bindings/pydrake/systems/primitives_py.cc b/bindings/pydrake/systems/primitives_py.cc
index 7e1129bd01e4..508cd05552f6 100644
--- a/bindings/pydrake/systems/primitives_py.cc
+++ b/bindings/pydrake/systems/primitives_py.cc
@@ -26,6 +26,7 @@
 #include "drake/systems/primitives/pass_through.h"
 #include "drake/systems/primitives/random_source.h"
 #include "drake/systems/primitives/saturation.h"
+#include "drake/systems/primitives/shared_pointer_system.h"
 #include "drake/systems/primitives/sine.h"
 #include "drake/systems/primitives/symbolic_vector_system.h"
 #include "drake/systems/primitives/trajectory_affine_system.h"
@@ -390,6 +391,35 @@ PYBIND11_MODULE(primitives, m) {
             doc.StateInterpolatorWithDiscreteDerivative.set_initial_position
                 .doc_2args_state_position);
 
+    DefineTemplateClassWithDefault<SharedPointerSystem<T>, LeafSystem<T>>(
+        m, "SharedPointerSystem", GetPyParam<T>(), doc.SharedPointerSystem.doc)
+        .def(py::init([](py::object value_to_hold) {
+          auto wrapped = std::make_unique<py::object>(std::move(value_to_hold));
+          return std::make_unique<SharedPointerSystem<T>>(std::move(wrapped));
+        }),
+            py::arg("value_to_hold"), doc.SharedPointerSystem.ctor.doc)
+        .def_static(
+            "AddToBuilder",
+            [](DiagramBuilder<T>* builder, py::object value_to_hold) {
+              auto wrapped =
+                  std::make_unique<py::object>(std::move(value_to_hold));
+              return SharedPointerSystem<T>::AddToBuilder(
+                  builder, std::move(wrapped));
+            },
+            py::arg("builder"), py::arg("value_to_hold"),
+            doc.SharedPointerSystem.AddToBuilder.doc)
+        .def(
+            "get",
+            [](const SharedPointerSystem<T>& self) {
+              py::object result = py::none();
+              py::object* held = self.template get<py::object>();
+              if (held != nullptr) {
+                result = std::move(*held);
+              }
+              return result;
+            },
+            doc.SharedPointerSystem.get.doc);
+
     DefineTemplateClassWithDefault<SymbolicVectorSystem<T>, LeafSystem<T>>(m,
         "SymbolicVectorSystem", GetPyParam<T>(), doc.SymbolicVectorSystem.doc)
         .def(py::init<std::optional<Variable>, VectorX<Variable>,
diff --git a/bindings/pydrake/systems/test/primitives_test.py b/bindings/pydrake/systems/test/primitives_test.py
index 036bb27fa539..ac64285f4ebd 100644
--- a/bindings/pydrake/systems/test/primitives_test.py
+++ b/bindings/pydrake/systems/test/primitives_test.py
@@ -45,6 +45,7 @@
     PerceptronActivationType,
     RandomSource,
     Saturation, Saturation_,
+    SharedPointerSystem, SharedPointerSystem_,
     Sine, Sine_,
     StateInterpolatorWithDiscreteDerivative,
     StateInterpolatorWithDiscreteDerivative_,
@@ -95,6 +96,7 @@ def test_instantiations(self):
         self._check_instantiations(MultilayerPerceptron_)
         self._check_instantiations(PassThrough_)
         self._check_instantiations(Saturation_)
+        self._check_instantiations(SharedPointerSystem_)
         self._check_instantiations(Sine_)
         self._check_instantiations(StateInterpolatorWithDiscreteDerivative_)
         self._check_instantiations(SymbolicVectorSystem_)
@@ -624,6 +626,26 @@ def test_ctor_api(self):
             period_sec=0.1,
             abstract_model_value=AbstractValue.Make("Hello world"))
 
+    def test_shared_pointer_system_ctor(self):
+        dut = SharedPointerSystem(value_to_hold=[1, 2, 3])
+        readback = dut.get()
+        self.assertListEqual(readback, [1, 2, 3])
+        del dut
+        self.assertListEqual(readback, [1, 2, 3])
+
+    def test_shared_pointer_system_builder(self):
+        builder = DiagramBuilder()
+        self.assertListEqual(
+            SharedPointerSystem.AddToBuilder(
+                builder=builder, value_to_hold=[1, 2, 3]),
+            [1, 2, 3])
+        diagram = builder.Build()
+        del builder
+        readback = diagram.GetSystems()[0].get()
+        self.assertListEqual(readback, [1, 2, 3])
+        del diagram
+        self.assertListEqual(readback, [1, 2, 3])
+
     def test_sine(self):
         # Test scalar output.
         sine_source = Sine(amplitude=1, frequency=2, phase=3,