Allow to use negative index of array in python (#2069)

* Allow to use negative index of array in python

* Support negative index in array slice

* Print index and array size in IndexError

* Fix style

python/tvm/container.py

@@ -17,10 +17,17 @@ class Array(NodeBase):
             start = i.start if i.start is not None else 0
             stop = i.stop if i.stop is not None else len(self)
             step = i.step if i.step is not None else 1
+            if start < 0:
+                start += len(self)
+            if stop < 0:
+                stop += len(self)
             return [self[idx] for idx in range(start, stop, step)]
 
-        if i >= len(self):
-            raise IndexError("array index out of range")
+        if i < -len(self) or i >= len(self):
+            raise IndexError("Array index out of range. Array size: {}, got index {}"
+                             .format(len(self), i))
+        if i < 0:
+            i += len(self)
         return _api_internal._ArrayGetItem(self, i)
 
     def __len__(self):

tests/python/unittest/test_lang_container.py

@@ -3,6 +3,9 @@ import tvm
 def test_array():
     a = tvm.convert([1,2,3])
     assert len(a) == 3
+    assert a[-1].value == 3
+    a_slice = a[-3:-1]
+    assert (a_slice[0].value, a_slice[1].value) == (1, 2)
 
 def test_array_save_load_json():
     a = tvm.convert([1,2,3])