object: handle nil arguments for topy to match CPython semantics

Fixes #61

Author: sbinet

object.go

@@ -20,6 +20,9 @@ func (self *PyObject) topy() *C.PyObject {
 }
 
 func topy(self *PyObject) *C.PyObject {
+	if self == nil {
+		return nil
+	}
 	return self.ptr
 }
 

python_test.go

@@ -130,3 +130,13 @@ ival=1666
 `),
 	})
 }
+
+func TestIssue61(t *testing.T) {
+	t.Parallel()
+	testPkg(t, pkg{
+		path: "tests/issue61",
+		want: []byte(`['i want this gone']
+[]
+`),
+	})
+}

sequence.go

@@ -323,8 +323,9 @@ func PyList_Append(self, item *PyObject) error {
 //
 // Changed in version 2.5: This function used an int for low and high. This might require changes in your code for properly supporting 64-bit systems.
 func PyList_SetSlice(self *PyObject, low, high int, itemlist *PyObject) error {
-	err := C.PyList_SetSlice(topy(self), C.Py_ssize_t(low), C.Py_ssize_t(high),
-		topy(itemlist))
+	err := C.PyList_SetSlice(
+		topy(self), C.Py_ssize_t(low), C.Py_ssize_t(high), topy(itemlist),
+	)
 	return int2err(err)
 }
 

tests/issue61/main.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/sbinet/go-python"
+)
+
+func init() {
+	err := python.Initialize()
+	if err != nil {
+		log.Panic(err)
+	}
+}
+
+func main() {
+
+	origList := python.PyList_New(0)
+	python.PyList_Append(origList, python.PyString_FromString("i want this gone"))
+	fmt.Println(python.PyString_AsString(origList.Str()))
+
+	python.PyList_SetSlice(origList, 0, 1, nil)
+	fmt.Println(python.PyString_AsString(origList.Str()))
+}