py: Fix range to support negative step

Author: corona10

py/range.go

@@ -67,6 +67,7 @@ func RangeNew(metatype *Type, args Tuple, kwargs StringDict) (Object, error) {
 	if err != nil {
 		return nil, err
 	}
+
 	return &Range{
 		Start: startIndex,
 		Stop:  stopIndex,
@@ -90,7 +91,11 @@ func (it *RangeIterator) M__iter__() (Object, error) {
 // Range iterator next
 func (it *RangeIterator) M__next__() (Object, error) {
 	r := it.Index
-	if r >= it.Stop {
+	if it.Step >= 0 && r >= it.Stop {
+		return nil, StopIteration
+	}
+
+	if it.Step < 0 && r <= it.Stop {
 		return nil, StopIteration
 	}
 	it.Index += it.Step

py/tests/range.py

@@ -0,0 +1,13 @@
+# Copyright 2018 The go-python Authors.  All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+doc="range"
+a = range(255)
+b = [e for e in a]
+assert len(b) == 255
+a = range(100, 0, -1)
+b = [e for e in a]
+assert len(b) == 100
+
+doc="finished"