Make indexing and slicing consistent with Python.

CHANGES.rst

@@ -30,6 +30,10 @@ Release History
 
 **Changed**
 
+- Integer indexing of Nengo objects out of range raises an ``IndexError``
+  now to be consistent with standard Python behaviour.
+  (`#1176 <https://github.com/nengo/nengo/issues/1176>`_,
+  `#1183 <https://github.com/nengo/nengo/pull/1183>`_)
 - Documentation that applies to all Nengo projects has been moved to
   https://nengo.github.io/.
   (`#1251 <https://github.com/nengo/nengo/pull/1251>`_)

nengo/base.py

@@ -148,28 +148,29 @@ class ObjView(object):
 
     def __init__(self, obj, key=slice(None)):
         self.obj = obj
-        if is_integer(key):
-            # single slices of the form [i] should be cast into
-            # slice objects for convenience
-            if key == -1:
-                # special case because slice(-1, 0) gives the empty list
-                key = slice(key, None)
-            else:
-                key = slice(key, key+1)
-        self.slice = key
 
         # Node.size_in != size_out, so one of these can be invalid
         try:
-            self.size_in = np.arange(self.obj.size_in)[self.slice].size
+            self.size_in = np.arange(self.obj.size_in)[key].size
         except IndexError:
             self.size_in = None
         try:
-            self.size_out = np.arange(self.obj.size_out)[self.slice].size
+            self.size_out = np.arange(self.obj.size_out)[key].size
         except IndexError:
             self.size_out = None
         if self.size_in is None and self.size_out is None:
-            raise ValidationError("Invalid slice '%s' of %s"
-                                  % (self.slice, self.obj), attr='key')
+            raise IndexError("Invalid slice '%s' of %s" % (key, self.obj))
+
+        if is_integer(key):
+            # single slices of the form [i] should be cast into
+            # slice objects for convenience
+            if key == -1:
+                # special case because slice(-1, 0) gives the empty list
+                self.slice = slice(key, None)
+            else:
+                self.slice = slice(key, key+1)
+        else:
+            self.slice = key
 
     def copy(self):
         return copy(self)

nengo/tests/test_ensemble.py

@@ -256,13 +256,28 @@ def test_len():
     assert len(ens1) == 1
     assert len(ens5) == 5
     assert len(ens1[0]) == 1
+    assert len(ens1[-1]) == 1
     assert len(ens5[:3]) == 3
+    assert len(ens5[:100]) == 5
 
     # Neurons.__len__
     assert len(ens1.neurons) == 10
     assert len(ens5.neurons) == 100
     assert len(ens1.neurons[0]) == 1
     assert len(ens5.neurons[90:]) == 10
+    assert len(ens5.neurons[90:150]) == 10
+
+
+def test_invalid_indices():
+    with nengo.Network():
+        ens = nengo.Ensemble(10, dimensions=2)
+
+    with pytest.raises(IndexError):
+        assert ens[5]
+    with pytest.raises(IndexError):
+        assert ens[-5]
+    with pytest.raises(IndexError):
+        assert ens.neurons[20]
 
 
 def test_invalid_rates(Simulator):