Add an example for sharing the memory buffer of the xarray between C++ and Python (numpy) (#175)

Demonstrate how to return an array and share the buffer to Python (numpy).

Author: yungyuc

.gitignore

@@ -39,6 +39,7 @@ test/CMakeCache.txt
 test/Makefile
 test/CMakeFiles/
 test/cmake_install.cmake
+.pytest_cache/
 
 # Documentation build artefacts
 docs/CMakeCache.txt

CMakeLists.txt

@@ -12,8 +12,8 @@ project(xtensor-python)
 set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake ${CMAKE_MODULE_PATH})
 set(XTENSOR_PYTHON_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/include)
 
-# Versionning
-# ===========
+# Versioning
+# ==========
 
 set(XTENSOR_PYTHON_CONFIG_FILE
     "${XTENSOR_PYTHON_INCLUDE_DIR}/xtensor-python/xtensor_python_config.hpp")

include/xtensor-python/xtensor_type_caster_base.hpp

@@ -39,7 +39,7 @@ namespace pybind11
             std::transform(src.strides().begin(), src.strides().end(),
                            python_strides.begin(), [](auto v) {
                 return sizeof(typename Type::value_type) * v;
-           });
+            });
 
             std::vector<std::size_t> python_shape(src.shape().size());
             std::copy(src.shape().begin(), src.shape().end(), python_shape.begin());
@@ -162,6 +162,8 @@ namespace pybind11
             {
                 return _("xt::xtensor");
             }
+#else
+            static constexpr auto name = _("xt::xtensor");
 #endif
 
             template <typename T>

test_python/main.cpp

@@ -122,6 +122,16 @@ struct B
     int b;
 };
 
+class C
+{
+public:
+    using array_type = xt::xarray<double, xt::layout_type::row_major>;
+    C() : m_array{0, 0, 0, 0} {}
+    array_type & array() { return m_array; }
+private:
+    array_type m_array;
+};
+
 xt::pyarray<A> dtype_to_python()
 {
     A a1{123, 321, 'a', {1, 2, 3}};
@@ -217,4 +227,16 @@ PYBIND11_MODULE(xtensor_python_test, m)
 
     m.def("col_major_array", col_major_array);
     m.def("row_major_tensor", row_major_tensor);
+
+    py::class_<C>(m, "C")
+        .def(py::init<>())
+        .def_property_readonly(
+            "copy",
+            [](C & self) { return self.array(); }
+        )
+        .def_property_readonly(
+            "ref",
+            [](C & self) -> C::array_type & { return self.array(); }
+        )
+    ;
 }

test_python/test_pyarray.py

@@ -146,3 +146,18 @@ def test_col_row_major(self):
         varF = np.arange(50, dtype=float).reshape(2, 5, 5, order='F')
         xt.col_major_array(varF)
         xt.col_major_array(varF[:, :, 0]) # still col major!
+
+class AttributeTest(TestCase):
+
+    def setUp(self):
+        self.c = xt.C()
+
+    def test_copy(self):
+        arr = self.c.copy
+        arr[0] = 1
+        self.assertEqual([0.]*4, self.c.copy.tolist())
+
+    def test_reference(self):
+        arr = self.c.ref
+        arr[0] = 1
+        self.assertEqual([1.] + [0.]*3, self.c.ref.tolist())