Create file if nonexistent with open mode 'ab+' (#1710)

Author: BCSharp

Src/IronPython/Modules/_fileio.cs

@@ -130,8 +130,9 @@ public FileIO(CodeContext/*!*/ context, string name, string mode = "r", bool clo
                             _readStream = _writeStream = OpenFile(context, pal, name, FileMode.CreateNew, FileAccess.ReadWrite, FileShare.ReadWrite);
                             break;
                         case "ab+":
-                            _readStream = OpenFile(context, pal, name, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
+                            // Opening _writeStream before _readStream will create the file if it does not exist
                             _writeStream = OpenFile(context, pal, name, FileMode.Append, FileAccess.Write, FileShare.ReadWrite);
+                            _readStream = OpenFile(context, pal, name, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
                             _readStream.Seek(0L, SeekOrigin.End);
                             _writeStream.Seek(0L, SeekOrigin.End);
                             break;

Tests/test_file.py

@@ -769,4 +769,10 @@ def test_opener_uncallable(self):
 
         self.assertRaises(TypeError, open, "", "r", opener=uncallable_opener)
 
+    def test_open_abplus(self):
+        with open(self.temp_file, "ab+") as f:
+            f.write(b"abc")
+            f.seek(0)
+            self.assertEqual(f.read(), b"abc")
+
 run_test(__name__)