BUG: (arg)sort and (arg)partition handling of empty multidim arrays

Fixes an error raised by the (arg)sorting and (arg)partitioning
functions, when called on an empty multidimensional array along
an axis that doesn't have 0 length.

2 files changed, 44 insertions, 3 deletions

diff --git a/numpy/core/src/multiarray/item_selection.c b/numpy/core/src/multiarray/item_selection.c
index 0d9474be3..a29c47555 100644
--- a/numpy/core/src/multiarray/item_selection.c
+++ b/numpy/core/src/multiarray/item_selection.c
@@ -817,7 +817,7 @@ _new_sortlike(PyArrayObject *op, int axis, PyArray_SortFunc *sort,
     NPY_BEGIN_THREADS_DEF;
 
     /* Check if there is any sorting to do */
-    if (N <= 1) {
+    if (N <= 1 || PyArray_SIZE(op) == 0) {
         return 0;
     }
 
@@ -964,7 +964,7 @@ _new_argsortlike(PyArrayObject *op, int axis, PyArray_ArgSortFunc *argsort,
     needidxbuffer = rstride != sizeof(npy_intp);
 
     /* Check if there is any argsorting to do */
-    if (N <= 1) {
+    if (N <= 1 || PyArray_SIZE(op) == 0) {
         memset(PyArray_DATA(rop), 0, PyArray_NBYTES(rop));
         return (PyObject *)rop;
     }
diff --git a/numpy/core/tests/test_multiarray.py b/numpy/core/tests/test_multiarray.py
index f6ed6dd84..7aadebccc 100644
--- a/numpy/core/tests/test_multiarray.py
+++ b/numpy/core/tests/test_multiarray.py
@@ -1029,6 +1029,15 @@ class TestMethods(TestCase):
         d.sort()
         assert_equal(d, c, "test sort with default axis")
 
+        # check axis handling for multidimensional empty arrays
+        a = np.array([])
+        a.shape = (3, 2, 1, 0)
+        for axis in range(-a.ndim, a.ndim):
+            assert_equal(np.sort(a, axis=axis), a,
+                         'test empty array sort with axis={}'.format(axis))
+        assert_equal(np.sort(a, axis=None), a.ravel(),
+                     'test empty array sort with axis=None')
+
     def test_copy(self):
         def assert_fortran(arr):
             assert_(arr.flags.fortran)
@@ -1189,7 +1198,6 @@ class TestMethods(TestCase):
             assert_equal(a.copy().argsort(kind=kind), r, msg)
             assert_equal(b.copy().argsort(kind=kind), rr, msg)
 
-
         # check axis handling. This should be the same for all type
         # specific argsorts, so we only check it for one type and one kind
         a = np.array([[3, 2], [1, 0]])
@@ -1201,6 +1209,17 @@ class TestMethods(TestCase):
         # using None is known fail at this point
         #assert_equal(a.copy().argsort(axis=None, c)
 
+        # check axis handling for multidimensional empty arrays
+        a = np.array([])
+        a.shape = (3, 2, 1, 0)
+        for axis in range(-a.ndim, a.ndim):
+            assert_equal(np.argsort(a, axis=axis),
+                         np.zeros_like(a, dtype=np.intp),
+                         'test empty array argsort with axis={}'.format(axis))
+        assert_equal(np.argsort(a, axis=None),
+                     np.zeros_like(a.ravel(), dtype=np.intp),
+                     'test empty array argsort with axis=None')
+
         # check that stable argsorts are stable
         r = np.arange(100)
         # scalars
@@ -1447,6 +1466,28 @@ class TestMethods(TestCase):
         assert_raises(ValueError, d_obj.partition, 10)
         assert_raises(ValueError, d_obj.partition, -11)
 
+    def test_partition_empty_array(self):
+        # check axis handling for multidimensional empty arrays
+        a = np.array([])
+        a.shape = (3, 2, 1, 0)
+        for axis in range(-a.ndim, a.ndim):
+            msg = 'test empty array partition with axis={}'.format(axis)
+            assert_equal(np.partition(a, 0, axis=axis), a, msg)
+        assert_equal(np.partition(a, 0, axis=None), a.ravel(),
+                     'test empty array partition with axis=None')
+
+    def test_argpartition_empty_array(self):
+        # check axis handling for multidimensional empty arrays
+        a = np.array([])
+        a.shape = (3, 2, 1, 0)
+        for axis in range(-a.ndim, a.ndim):
+            msg = 'test empty array argpartition with axis={}'.format(axis)
+            assert_equal(np.partition(a, 0, axis=axis),
+                         np.zeros_like(a, dtype=np.intp), msg)
+        assert_equal(np.partition(a, 0, axis=None),
+                     np.zeros_like(a.ravel(), dtype=np.intp),
+                     'test empty array argpartition with axis=None')
+
     def test_partition(self):
         d = np.arange(10)
         assert_raises(TypeError, np.partition, d, 2, kind=1)