ENH: add a .ndim property to dype objects

This complements the .shape property

4 files changed, 53 insertions, 0 deletions

diff --git a/doc/release/1.13.0-notes.rst b/doc/release/1.13.0-notes.rst
index ae3c64fdd..03e5f955a 100644
--- a/doc/release/1.13.0-notes.rst
+++ b/doc/release/1.13.0-notes.rst
@@ -159,6 +159,11 @@ to `apply_along_axis`. Now, it can return an array of any dimensionality
 (including 0D), and the shape of this array replaces the axis of the array
 being iterated over.
 
+``.ndim`` property added to ``dtype`` to complement ``.shape``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+For consistency with ``ndarray`` and ``broadcast``, ``d.ndim`` is a shorthand
+for ``len(d.shape)``.
+
 
 Changes
 =======
diff --git a/numpy/add_newdocs.py b/numpy/add_newdocs.py
index 3916d1304..cf4af9f84 100644
--- a/numpy/add_newdocs.py
+++ b/numpy/add_newdocs.py
@@ -6355,6 +6355,15 @@ add_newdoc('numpy.core.multiarray', 'dtype', ('shape',
 
     """))
 
+add_newdoc('numpy.core.multiarray', 'dtype', ('ndim',
+    """
+    Number of dimensions of the sub-array if this data type describes a
+    sub-array, and ``0`` otherwise.
+
+    .. versionadded:: 1.13.0
+
+    """))
+
 add_newdoc('numpy.core.multiarray', 'dtype', ('str',
     """The array-protocol typestring of this data-type object."""))
 
diff --git a/numpy/core/src/multiarray/descriptor.c b/numpy/core/src/multiarray/descriptor.c
index 6c2186f9c..86da751ba 100644
--- a/numpy/core/src/multiarray/descriptor.c
+++ b/numpy/core/src/multiarray/descriptor.c
@@ -1950,6 +1950,25 @@ arraydescr_shape_get(PyArray_Descr *self)
     return Py_BuildValue("(O)", self->subarray->shape);
 }
 
+static PyObject *
+arraydescr_ndim_get(PyArray_Descr *self)
+{
+    if (!PyDataType_HASSUBARRAY(self)) {
+        return PyInt_FromLong(0);
+    }
+    /*TODO
+     * self->subarray->shape should always be a tuple,
+     * so this check should be unnecessary
+     */
+    if (PyTuple_Check(self->subarray->shape)) {
+        Py_ssize_t ndim = PyTuple_Size(self->subarray->shape);
+        return PyInt_FromLong(ndim);
+    }
+    /* consistent with arraydescr_shape_get */
+    return PyInt_FromLong(1);
+}
+
+
 NPY_NO_EXPORT PyObject *
 arraydescr_protocol_descr_get(PyArray_Descr *self)
 {
@@ -2208,6 +2227,9 @@ static PyGetSetDef arraydescr_getsets[] = {
     {"shape",
         (getter)arraydescr_shape_get,
         NULL, NULL, NULL},
+    {"ndim",
+        (getter)arraydescr_ndim_get,
+        NULL, NULL, NULL},
     {"isbuiltin",
         (getter)arraydescr_isbuiltin_get,
         NULL, NULL, NULL},
diff --git a/numpy/core/tests/test_dtype.py b/numpy/core/tests/test_dtype.py
index c0143aae3..f8e9b899d 100644
--- a/numpy/core/tests/test_dtype.py
+++ b/numpy/core/tests/test_dtype.py
@@ -376,6 +376,23 @@ class TestSubarray(TestCase):
         assert_(isinstance(dt['a'].shape, tuple))
         assert_(isinstance(dt['a'].shape[0], int))
 
+    def test_shape_matches_ndim(self):
+        dt = np.dtype([('a', 'f4', ())])
+        assert_equal(dt['a'].shape, ())
+        assert_equal(dt['a'].ndim, 0)
+
+        dt = np.dtype([('a', 'f4')])
+        assert_equal(dt['a'].shape, ())
+        assert_equal(dt['a'].ndim, 0)
+
+        dt = np.dtype([('a', 'f4', 4)])
+        assert_equal(dt['a'].shape, (4,))
+        assert_equal(dt['a'].ndim, 1)
+
+        dt = np.dtype([('a', 'f4', (1, 2, 3))])
+        assert_equal(dt['a'].shape, (1, 2, 3))
+        assert_equal(dt['a'].ndim, 3)
+
     def test_shape_invalid(self):
         # Check that the shape is valid.
         max_int = np.iinfo(np.intc).max