Add Slice function for range type

Crate a new range object by calculating start,
stop, and step when slice is entered as argument
to the __getitem__ function of the range

Fixes #77

Author: HyeockJinKim

py/range.go

@@ -79,12 +79,54 @@ func RangeNew(metatype *Type, args Tuple, kwargs StringDict) (Object, error) {
 }
 
 func (r *Range) M__getitem__(key Object) (Object, error) {
+	length := r.Length
+	if slice, ok := key.(*Slice); ok {
+		length, err := length.GoInt()
+		if err != nil {
+			return nil, err
+		}
+		start, stop, step, sliceLength, err := slice.GetIndices(length)
+		if err != nil {
+			return nil, err
+		}
+
+		if step == 0 {
+			return nil, ExceptionNewf(ValueError, "slice step cannot be zero")
+		}
+		if start < 0 {
+			start += length
+		}
+		if stop < 0 {
+			stop += length
+		}
+		if start < 0 {
+			start = 0
+		} else if start >= length {
+			start = length
+		}
+		if stop < 0 {
+			stop = 0
+		} else if stop >= length {
+			stop = length
+		}
+
+		startIndex := computeItem(r, Int(start))
+		stopIndex := computeItem(r, Int(stop))
+		stepIndex := Int(step) * r.Step
+
+		return &Range{
+			Start: startIndex,
+			Stop: stopIndex,
+			Step: stepIndex,
+			Length: Int(sliceLength),
+		}, nil
+	}
+
 	index, err := Index(key)
 	if err != nil {
 		return nil, err
 	}
-	// TODO(corona10): Support slice case
-	length := computeRangeLength(r.Start, r.Stop, r.Step)
+
 	if index < 0 {
 		index += length
 	}