diff --git a/examples/ch2-07/hello-py/Makefile b/examples/ch2-07/hello-py/Makefile
index 69a2cc0..2c0e6a6 100644
--- a/examples/ch2-07/hello-py/Makefile
+++ b/examples/ch2-07/hello-py/Makefile
@@ -1,6 +1,7 @@
 default:
 	go build -buildmode=c-shared -o gopkg.so main.go
 	python3 -c 'import gopkg; print(gopkg.system("time"))'
+	python3 -c 'import gopkg; print(gopkg.sum(2, 3))'
 
 clean:
 	-rm *.so
diff --git a/examples/ch2-07/hello-py/gopkg.h b/examples/ch2-07/hello-py/gopkg.h
new file mode 100644
index 0000000..a5f4a5a
--- /dev/null
+++ b/examples/ch2-07/hello-py/gopkg.h
@@ -0,0 +1,106 @@
+/* Created by "go tool cgo" - DO NOT EDIT. */
+
+/* package command-line-arguments */
+
+/* Start of preamble from import "C" comments.  */
+
+
+#line 3 "/Users/chai/go/src/github.com/chai2010/advanced-go-programming-book/examples/ch2-07/hello-py/main.go"
+
+// macOS:
+// python3-config --cflags
+// python3-config --ldflags
+
+
+
+// linux
+
+
+// windows
+// should generate libpython3.a from python3.lib
+
+#define Py_LIMITED_API
+#include <Python.h>
+
+static PyObject *
+spam_system(PyObject *self, PyObject *args) {
+	const char *command;
+	if (!PyArg_ParseTuple(args, "s", &command)) {
+		return NULL;
+	}
+
+	int status = system(command);
+	return PyLong_FromLong(status);
+}
+
+extern PyObject *sum(PyObject *self, PyObject *args);
+
+static PyMethodDef modMethods[] = {
+	{"system",  spam_system, METH_VARARGS, "Execute a shell command."},
+	{"sum",  sum, METH_VARARGS, "Execute a shell command."},
+	{NULL, NULL, 0, NULL}
+};
+
+static PyObject* PyInit_gopkg_(void) {
+	static struct PyModuleDef module = {
+		PyModuleDef_HEAD_INIT, "gopkg", NULL, -1, modMethods,
+	};
+	return (void*)PyModule_Create(&module);
+}
+
+#line 1 "cgo-generated-wrapper"
+
+
+/* End of preamble from import "C" comments.  */
+
+
+/* Start of boilerplate cgo prologue.  */
+#line 1 "cgo-gcc-export-header-prolog"
+
+#ifndef GO_CGO_PROLOGUE_H
+#define GO_CGO_PROLOGUE_H
+
+typedef signed char GoInt8;
+typedef unsigned char GoUint8;
+typedef short GoInt16;
+typedef unsigned short GoUint16;
+typedef int GoInt32;
+typedef unsigned int GoUint32;
+typedef long long GoInt64;
+typedef unsigned long long GoUint64;
+typedef GoInt64 GoInt;
+typedef GoUint64 GoUint;
+typedef __SIZE_TYPE__ GoUintptr;
+typedef float GoFloat32;
+typedef double GoFloat64;
+typedef float _Complex GoComplex64;
+typedef double _Complex GoComplex128;
+
+/*
+  static assertion to make sure the file is being used on architecture
+  at least with matching size of GoInt.
+*/
+typedef char _check_for_64_bit_pointer_matching_GoInt[sizeof(void*)==64/8 ? 1:-1];
+
+typedef struct { const char *p; GoInt n; } GoString;
+typedef void *GoMap;
+typedef void *GoChan;
+typedef struct { void *t; void *v; } GoInterface;
+typedef struct { void *data; GoInt len; GoInt cap; } GoSlice;
+
+#endif
+
+/* End of boilerplate cgo prologue.  */
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+extern PyObject* sum(PyObject* p0, PyObject* p1);
+
+extern PyObject* PyInit_gopkg();
+
+#ifdef __cplusplus
+}
+#endif
diff --git a/examples/ch2-07/hello-py/main.go b/examples/ch2-07/hello-py/main.go
index 46f2977..737317b 100644
--- a/examples/ch2-07/hello-py/main.go
+++ b/examples/ch2-07/hello-py/main.go
@@ -27,12 +27,15 @@ spam_system(PyObject *self, PyObject *args) {
 	return PyLong_FromLong(status);
 }
 
+extern PyObject *sum(PyObject *self, PyObject *args);
+
 static PyMethodDef modMethods[] = {
 	{"system",  spam_system, METH_VARARGS, "Execute a shell command."},
+	{"sum",  sum, METH_VARARGS, "Execute a shell command."},
 	{NULL, NULL, 0, NULL}
 };
 
-void* PyInit_gopkg(void) {
+static PyObject* PyInit_gopkg_(void) {
 	static struct PyModuleDef module = {
 		PyModuleDef_HEAD_INIT, "gopkg", NULL, -1, modMethods,
 	};
@@ -41,7 +44,9 @@ void* PyInit_gopkg(void) {
 */
 import "C"
 
-import "fmt"
+import (
+	"fmt"
+)
 
 func main() {}
 
@@ -49,3 +54,13 @@ func main() {}
 func SayHello(name *C.char) {
 	fmt.Printf("hello %s!\n", C.GoString(name))
 }
+
+//export sum
+func sum(self, args *C.PyObject) *C.PyObject {
+	return C.PyLong_FromLongLong(9527) // TODO
+}
+
+//export PyInit_gopkg
+func PyInit_gopkg() *C.PyObject {
+	return C.PyInit_gopkg_()
+}