py: handle nil module pointers in RunCode

py/run.go

@@ -143,7 +143,7 @@ func RunSrc(ctx Context, pySrc string, pySrcDesc string, inModule interface{}) (
 //
 // If inModule is a *Module, then the code is run in that module.
 //
-// If inModule is nil, the code is run in a new __main__ module (and the new Module is returned).
+// If inModule is nil or a nil *Module, the code is run in a new __main__ module (and the new Module is returned).
 //
 // If inModule is a string, the code is run in a new module with the given name (and the new Module is returned).
 func RunCode(ctx Context, code *Code, codeDesc string, inModule interface{}) (*Module, error) {
@@ -162,6 +162,10 @@ func RunCode(ctx Context, code *Code, codeDesc string, inModule interface{}) (*M
 	case nil:
 		createNew = true
 	case *Module:
+		if mod == nil {
+			createNew = true
+			break
+		}
 		_, err = ctx.RunCode(code, mod.Globals, mod.Globals, nil)
 		module = mod
 	default:

py/run_test.go

@@ -0,0 +1,54 @@
+// Copyright 2026 The go-python Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package py_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/go-python/gpython/py"
+	_ "github.com/go-python/gpython/stdlib"
+)
+
+func TestRunWithNilModulePointer(t *testing.T) {
+	for _, entry := range []string{"RunCode", "RunSrc", "RunFile"} {
+		t.Run(entry, func(t *testing.T) {
+			ctx := py.NewContext(py.DefaultContextOpts())
+			defer ctx.Close()
+			src := "value = 42\n"
+			var module *py.Module
+			var err error
+			switch entry {
+			case "RunCode":
+				var code *py.Code
+				code, err = py.Compile(src, "<test>", py.ExecMode, 0, true)
+				if err == nil {
+					module, err = py.RunCode(ctx, code, "<test>", module)
+				}
+			case "RunSrc":
+				module, err = py.RunSrc(ctx, src, "<test>", module)
+			case "RunFile":
+				path := filepath.Join(t.TempDir(), "test.py")
+				if err = os.WriteFile(path, []byte(src), 0600); err == nil {
+					module, err = py.RunFile(ctx, filepath.Base(path), py.CompileOpts{CurDir: filepath.Dir(path)}, module)
+				}
+			}
+			if err != nil {
+				t.Fatal(err)
+			}
+			if module == nil || module.Globals["value"] != py.Int(42) {
+				t.Fatalf("expected a new module containing value = 42, got %v", module)
+			}
+			reused, err := py.RunSrc(ctx, "value += 1", "<reuse>", module)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if reused != module || module.Globals["value"] != py.Int(43) {
+				t.Fatal("expected the existing module and globals to be reused")
+			}
+		})
+	}
+}