DEP: Deprecate incorrect behavior of expand_dims.

Expand_dims works as documented when the index of the inserted NewAxis
in the resulting array satisfies -a.ndim - 1 <= index <= a.ndim.
However, when index > a.ndim index is replaced by a.ndim and, when
index < -a.ndim - 1, it is replaced by index + a.ndim + 1, which may be
negative and results in incorrect placement. The latter two cases are
now deprecated.

Closes #9100.

1 files changed, 22 insertions, 1 deletions

diff --git a/numpy/lib/tests/test_shape_base.py b/numpy/lib/tests/test_shape_base.py
index 4d06001f4..14406fe21 100644
--- a/numpy/lib/tests/test_shape_base.py
+++ b/numpy/lib/tests/test_shape_base.py
@@ -1,9 +1,11 @@
 from __future__ import division, absolute_import, print_function
 
 import numpy as np
+import warnings
+
 from numpy.lib.shape_base import (
     apply_along_axis, apply_over_axes, array_split, split, hsplit, dsplit,
-    vsplit, dstack, column_stack, kron, tile
+    vsplit, dstack, column_stack, kron, tile, expand_dims,
     )
 from numpy.testing import (
     run_module_suite, TestCase, assert_, assert_equal, assert_array_equal,
@@ -182,6 +184,25 @@ class TestApplyOverAxes(TestCase):
         assert_array_equal(aoa_a, np.array([[[60], [92], [124]]]))
 
 
+class TestExpandDims(TestCase):
+    def test_functionality(self):
+        s = (2, 3, 4, 5)
+        a = np.empty(s)
+        for axis in range(-5, 4):
+            b = expand_dims(a, axis)
+            assert_(b.shape[axis] == 1)
+            assert_(np.squeeze(b).shape == s)
+
+    def test_deprecations(self):
+        # 2017-05-17, 1.13.0
+        s = (2, 3, 4, 5)
+        a = np.empty(s)
+        with warnings.catch_warnings():
+            warnings.simplefilter("always")
+            assert_warns(DeprecationWarning, expand_dims, a, -6)
+            assert_warns(DeprecationWarning, expand_dims, a, 5)
+
+
 class TestArraySplit(TestCase):
     def test_integer_0_split(self):
         a = np.arange(10)