Changed the behavior of getsize() on buffered append to match real OS

- fixes pytest-dev#281

fake_filesystem_test.py

@@ -3462,7 +3462,7 @@ def testFileSizeAfterWrite(self):
         expected_size = original_size + len(added_content)
         fh = self.open(file_path, 'a')
         fh.write(added_content)
-        self.assertEqual(expected_size, fh.Size())
+        self.assertEqual(original_size, fh.Size())
         fh.close()
         self.assertEqual(expected_size, self.open(file_path, 'r').Size())
 
@@ -3473,13 +3473,8 @@ def testLargeFileSizeAfterWrite(self):
         self.filesystem.CreateFile(file_path, st_size=original_size)
         added_content = 'foo bar'
         fh = self.open(file_path, 'a')
-        # We can't use assertRaises, because the exception is thrown
-        # in __getattr__, so just saying 'fh.write' causes the exception.
-        try:
-            fh.write(added_content)
-        except fake_filesystem.FakeLargeFileIoException:
-            return
-        self.fail('Writing to a large file should not be allowed')
+        self.assertRaises(fake_filesystem.FakeLargeFileIoException,
+                          lambda: fh.write(added_content))
 
     def testFileSizeUpdatedViaFlush(self):
         """test that file size gets updated via flush()."""

pyfakefs/fake_filesystem.py

@@ -4255,7 +4255,6 @@ def other_wrapper(*args, **kwargs):
             if write_seek != self._io.tell():
                 self._read_seek = self._io.tell()
                 self._read_whence = 0
-                self._file_object.st_size += (self._read_seek - write_seek)
             return ret_value
 
         return other_wrapper