resolve go-python#169 (comment)

Author: glaukiol1

os/os.module.go

@@ -102,29 +102,25 @@ func getenv(self py.Object, args py.Tuple) (py.Object, error) {
 		dflt py.Object = py.None
 		err  error
 	)
+	if len(args) == 0 {
+		return nil, py.ExceptionNewf(py.TypeError, "missing one required argument: 'name:str'")
+	}
+	if !objectIsString(args[0]) {
+		return nil, py.ExceptionNewf(py.TypeError, "str expected (pos 1), not "+args[0].Type().Name)
+	}
 	switch len(args) {
 	case 1:
-		if objectIsString(args[0]) {
-			key = args[0]
-		} else {
-			return nil, py.ExceptionNewf(py.TypeError, "str expected (pos 1), not "+args[0].Type().Name)
-		}
+		key = args[0]
 		dflt = py.None
 		break
 	case 2:
-		if objectIsString(args[0]) {
-			key = args[0]
-		} else {
-			return nil, py.ExceptionNewf(py.TypeError, "str expected (pos 1), not "+args[0].Type().Name)
-		}
+		key = args[0]
 		if objectIsString(args[1]) {
 			dflt = args[1]
 		} else {
 			return nil, py.ExceptionNewf(py.TypeError, "str expected (pos 2), not "+args[1].Type().Name)
 		}
 		break
-	default:
-		return nil, py.ExceptionNewf(py.TypeError, "missing one required argument: 'name:str'")
 	}
 	gokey, err := py.ReprAsString(key)
 	if err != nil {