Add function for comparing buffer_info formats to types

Allows equivalent integral types and numpy dtypes

Author: patstew

include/pybind11/common.h

@@ -617,6 +617,24 @@ template <typename T> struct format_descriptor<T, detail::enable_if_t<detail::is
 template <typename T> constexpr const char format_descriptor<
     T, detail::enable_if_t<detail::is_fmt_numeric<T>::value>>::value[2];
 
+NAMESPACE_BEGIN(detail)
+
+template <typename T, typename SFINAE = void> struct compare_buffer_info {
+    static bool compare(const buffer_info& b) {
+        return b.format == format_descriptor<T>::format() && b.itemsize == sizeof(T);
+    }
+};
+
+template <typename T> struct compare_buffer_info<T, detail::enable_if_t<std::is_integral<T>::value>> {
+    static bool compare(const buffer_info& b) {
+        return b.itemsize == sizeof(T) && (b.format == format_descriptor<T>::value ||
+            ((sizeof(T) == sizeof(long)) && b.format == (std::is_unsigned<T>::value ? "L" : "l")) ||
+            ((sizeof(T) == sizeof(size_t)) && b.format == (std::is_unsigned<T>::value ? "N" : "n")));
+    }
+};
+
+NAMESPACE_END(detail)
+
 /// RAII wrapper that temporarily clears any Python error state
 struct error_scope {
     PyObject *type, *value, *trace;

include/pybind11/numpy.h

@@ -694,6 +694,13 @@ struct pyobject_caster<array_t<T, ExtraFlags>> {
     PYBIND11_TYPE_CASTER(type, handle_type_name<type>::name());
 };
 
+template <typename T>
+struct compare_buffer_info<T, detail::enable_if_t<detail::is_pod_struct<T>::value>> {
+    static bool compare(const buffer_info& b) {
+        return npy_api::get().PyArray_EquivTypes_(dtype::of<T>().ptr(), dtype(b).ptr());
+    }
+};
+
 template <typename T> struct npy_format_descriptor<T, enable_if_t<satisfies_any_of<T, std::is_arithmetic, is_complex>::value>> {
 private:
     // NB: the order here must match the one in common.h

tests/test_numpy_dtypes.cpp

@@ -319,6 +319,22 @@ py::list test_dtype_methods() {
     return list;
 }
 
+struct CompareStruct {
+    bool x;
+    uint32_t y;
+    float z;
+};
+
+py::list test_compare_buffer_info() {
+    py::list list;
+    list.append(py::bool_(py::detail::compare_buffer_info<float>::compare(py::buffer_info(nullptr, sizeof(float), "f", 1))));
+    list.append(py::bool_(py::detail::compare_buffer_info<unsigned>::compare(py::buffer_info(nullptr, sizeof(int), "I", 1))));
+    list.append(py::bool_(py::detail::compare_buffer_info<long>::compare(py::buffer_info(nullptr, sizeof(long), "l", 1))));
+    list.append(py::bool_(py::detail::compare_buffer_info<long>::compare(py::buffer_info(nullptr, sizeof(long), sizeof(long) == sizeof(int) ? "i" : "q", 1))));
+    list.append(py::bool_(py::detail::compare_buffer_info<CompareStruct>::compare(py::buffer_info(nullptr, sizeof(CompareStruct), "T{?:x:3xI:y:f:z:}", 1))));
+    return list;
+}
+
 test_initializer numpy_dtypes([](py::module &m) {
     try {
         py::module::import("numpy");
@@ -337,6 +353,7 @@ test_initializer numpy_dtypes([](py::module &m) {
     PYBIND11_NUMPY_DTYPE(StringStruct, a, b);
     PYBIND11_NUMPY_DTYPE(EnumStruct, e1, e2);
     PYBIND11_NUMPY_DTYPE(TrailingPaddingStruct, a, b);
+    PYBIND11_NUMPY_DTYPE(CompareStruct, x, y, z);
 
     // ... or after
     py::class_<PackedStruct>(m, "PackedStruct");
@@ -366,6 +383,7 @@ test_initializer numpy_dtypes([](py::module &m) {
     m.def("test_array_ctors", &test_array_ctors);
     m.def("test_dtype_ctors", &test_dtype_ctors);
     m.def("test_dtype_methods", &test_dtype_methods);
+    m.def("compare_buffer_info", &test_compare_buffer_info);
     m.def("trailing_padding_dtype", &trailing_padding_dtype);
     m.def("buffer_to_dtype", &buffer_to_dtype);
     m.def("f_simple", [](SimpleStruct s) { return s.uint_ * 10; });

tests/test_numpy_dtypes.py

@@ -263,3 +263,9 @@ def test_register_dtype():
     with pytest.raises(RuntimeError) as excinfo:
         register_dtype()
     assert 'dtype is already registered' in str(excinfo.value)
+
+
+@pytest.requires_numpy
+def test_compare_buffer_info():
+    from pybind11_tests import compare_buffer_info
+    assert all(compare_buffer_info())

tests/test_stl_binders.py

@@ -55,7 +55,7 @@ def test_vector_buffer():
 
 @pytest.requires_numpy
 def test_vector_buffer_numpy():
-    from pybind11_tests import VectorInt, get_vectorstruct
+    from pybind11_tests import VectorInt, VectorStruct, get_vectorstruct
 
     a = np.array([1, 2, 3, 4], dtype=np.int32)
     with pytest.raises(TypeError):
@@ -79,6 +79,10 @@ def test_vector_buffer_numpy():
     m[1]['x'] = 99
     assert v[1].x == 99
 
+    v = VectorStruct(np.zeros(3, dtype=np.dtype([('w', 'bool'), ('x', 'I'),
+                                                 ('y', 'float64'), ('z', 'bool')], align=True)))
+    assert len(v) == 3
+
 
 def test_vector_custom():
     from pybind11_tests import El, VectorEl, VectorVectorEl