view for numpy arrays (#987)

* reshape

* more tests

* Update numpy.h

* Update test_numpy_array.py

* array view

* test

* Update test_numpy_array.cpp

* Update numpy.h

* Update numpy.h

* Update test_numpy_array.cpp

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Fix merge bug

* Make clang-tidy happy

* Add xfail for PyPy

* Fix casting issue

* Fix formatting

* Apply clang-tidy

* Address reviews on additional tests

* Fix ordering

* Do a little more reordering

* Fix typo

* Try improving tests

* Fix error in reshape

* Add one more reshape test

* Fix bugs and add test

* Relax test

* streamlining new tests; removing a few stray msg

* Fix style revert

* Fix clang-tidy

* Misc tweaks:
* Comment: matching style in file (///), responsibility sentence, consistent punctuation.
* Replacing `unsigned char` with `uint8_t` for max consistency.
* Removing `1` from `array_view1` because there is only one.

* Partial clang-format-diff.

Co-authored-by: ncullen93 <ncullen.th@dartmouth.edu>
Co-authored-by: NC Cullen <nicholas.c.cullen.th@dartmouth.edu>
Co-authored-by: Aaron Gokaslan <skylion.aaron@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Ralf Grosse-Kunstleve <rwgk@google.com>

include/pybind11/numpy.h

@@ -199,6 +199,7 @@ struct npy_api {
     int (*PyArray_SetBaseObject_)(PyObject *, PyObject *);
     PyObject* (*PyArray_Resize_)(PyObject*, PyArray_Dims*, int, int);
     PyObject* (*PyArray_Newshape_)(PyObject*, PyArray_Dims*, int);
+    PyObject* (*PyArray_View_)(PyObject*, PyObject*, PyObject*);
 
 private:
     enum functions {
@@ -216,6 +217,7 @@ private:
         API_PyArray_DescrNewFromType = 96,
         API_PyArray_Newshape = 135,
         API_PyArray_Squeeze = 136,
+        API_PyArray_View = 137,
         API_PyArray_DescrConverter = 174,
         API_PyArray_EquivTypes = 182,
         API_PyArray_GetArrayParamsFromObject = 278,
@@ -248,6 +250,7 @@ private:
         DECL_NPY_API(PyArray_DescrNewFromType);
         DECL_NPY_API(PyArray_Newshape);
         DECL_NPY_API(PyArray_Squeeze);
+        DECL_NPY_API(PyArray_View);
         DECL_NPY_API(PyArray_DescrConverter);
         DECL_NPY_API(PyArray_EquivTypes);
         DECL_NPY_API(PyArray_GetArrayParamsFromObject);
@@ -802,6 +805,21 @@ public:
         return new_array;
     }
 
+    /// Create a view of an array in a different data type.
+    /// This function may fundamentally reinterpret the data in the array.
+    /// It is the responsibility of the caller to ensure that this is safe.
+    /// Only supports the `dtype` argument, the `type` argument is omitted,
+    /// to be added as needed.
+    array view(const std::string &dtype) {
+        auto &api = detail::npy_api::get();
+        auto new_view = reinterpret_steal<array>(api.PyArray_View_(
+            m_ptr, dtype::from_args(pybind11::str(dtype)).release().ptr(), nullptr));
+        if (!new_view) {
+            throw error_already_set();
+        }
+        return new_view;
+    }
+
     /// Ensure that the argument is a NumPy array
     /// In case of an error, nullptr is returned and the Python error is cleared.
     static array ensure(handle h, int ExtraFlags = 0) {

tests/test_numpy_array.cpp

@@ -405,6 +405,9 @@ TEST_SUBMODULE(numpy_array, sm) {
         return a;
     });
 
+    sm.def("array_view",
+           [](py::array_t<uint8_t> a, const std::string &dtype) { return a.view(dtype); });
+
     sm.def("reshape_initializer_list", [](py::array_t<int> a, size_t N, size_t M, size_t O) {
         return a.reshape({N, M, O});
     });

tests/test_numpy_array.py

@@ -476,6 +476,21 @@ def test_array_create_and_resize():
     assert np.all(a == 42.0)
 
 
+def test_array_view():
+    a = np.ones(100 * 4).astype("uint8")
+    a_float_view = m.array_view(a, "float32")
+    assert a_float_view.shape == (100 * 1,)  # 1 / 4 bytes = 8 / 32
+
+    a_int16_view = m.array_view(a, "int16")  # 1 / 2 bytes = 16 / 32
+    assert a_int16_view.shape == (100 * 2,)
+
+
+def test_array_view_invalid():
+    a = np.ones(100 * 4).astype("uint8")
+    with pytest.raises(TypeError):
+        m.array_view(a, "deadly_dtype")
+
+
 def test_reshape_initializer_list():
     a = np.arange(2 * 7 * 3) + 1
     x = m.reshape_initializer_list(a, 2, 7, 3)