Implement unary negation slot.

compiler/expr_visitor.py

@@ -371,6 +371,7 @@ class ExprVisitor(ast.NodeVisitor):
 
   _UNARY_OP_TEMPLATES = {
       ast.Invert: 'πg.Invert(πF, {operand})',
+      ast.USub: 'πg.Neg(πF, {operand})',
   }
 
   def visit_function_inline(self, node):

runtime/core.go

@@ -759,6 +759,16 @@ func ToStr(f *Frame, o *Object) (*Str, *BaseException) {
 	return toStrUnsafe(result), nil
 }
 
+// Neg returns the result of o.__neg__ and is equivalent to the Python
+// expression "-o".
+func Neg(f *Frame, o *Object) (*Object, *BaseException) {
+	neg := o.typ.slots.Neg
+	if neg == nil {
+		return nil, f.RaiseType(TypeErrorType, fmt.Sprintf("bad operand type for unary ~: '%s'", o.typ.Name()))
+	}
+	return neg.Fn(f, o)
+}
+
 // Xor returns the result of the bitwise xor operator v ^ w according to
 // __xor/rxor__.
 func Xor(f *Frame, v, w *Object) (*Object, *BaseException) {

runtime/slots.go

@@ -408,6 +408,7 @@ type typeSlots struct {
 	Mul          *binaryOpSlot
 	Native       *nativeSlot
 	NE           *binaryOpSlot
+	Neg          *unaryOpSlot
 	New          *newSlot
 	Next         *unaryOpSlot
 	NonZero      *unaryOpSlot