Expose some dtype/array attributes via NumPy C API

tests/test_numpy_array.py

@@ -0,0 +1,43 @@
+import pytest
+
+with pytest.suppress(ImportError):
+    import numpy as np
+
+
+@pytest.requires_numpy
+def test_array_attributes():
+    from pybind11_tests import (get_arr_ndim, get_arr_shape, get_arr_strides, get_arr_writeable,
+                                get_arr_size, get_arr_itemsize, get_arr_nbytes, get_arr_owndata)
+
+    a = np.array(0, 'f8')
+    assert get_arr_ndim(a) == 0
+    assert get_arr_shape(a) == []
+    assert get_arr_strides(a) == []
+    with pytest.raises(RuntimeError):
+        get_arr_shape(a, 1)
+    with pytest.raises(RuntimeError):
+        get_arr_strides(a, 0)
+    assert get_arr_writeable(a)
+    assert get_arr_size(a) == 1
+    assert get_arr_itemsize(a) == 8
+    assert get_arr_nbytes(a) == 8
+    assert get_arr_owndata(a)
+
+    a = np.array([[1, 2, 3], [4, 5, 6]], 'u2').view()
+    a.flags.writeable = False
+    assert get_arr_ndim(a) == 2
+    assert get_arr_shape(a) == [2, 3]
+    assert get_arr_shape(a, 0) == 2
+    assert get_arr_shape(a, 1) == 3
+    assert get_arr_strides(a) == [6, 2]
+    assert get_arr_strides(a, 0) == 6
+    assert get_arr_strides(a, 1) == 2
+    with pytest.raises(RuntimeError):
+        get_arr_shape(a, 2)
+    with pytest.raises(RuntimeError):
+        get_arr_strides(a, 2)
+    assert not get_arr_writeable(a)
+    assert get_arr_size(a) == 6
+    assert get_arr_itemsize(a) == 2
+    assert get_arr_nbytes(a) == 12
+    assert not get_arr_owndata(a)