some improvements

src/file_transfers.py

@@ -18,29 +18,30 @@ class SendTransfer(FileTransfer):
     def __init__(self, path, tox, friend_number):
         super(self.__class__, self).__init__(path, tox, friend_number)
         self._file_number = tox.file_send(friend_number, TOX_FILE_KIND['DATA'], getsize(path), None, basename(path))
+        self._file = open(path, 'rb')
 
     def get_file_number(self):
         return self._file_number
 
     def send_chunk(self, position, size):
-        with open(self._path) as f:
-            f.seek(position)
-            data = f.read(size)
+        self._file.seek(position)
+        data = self._file.read(size)
         return self._tox.file_send_chunk(self._friend_number, self._file_number, position, data)
 
 
 class ReceiveTransfer(FileTransfer):
     def __init__(self, path, tox, friend_number):
         super(self.__class__, self).__init__(path, tox, friend_number)
+        self._file = open(self._path, 'wb')
 
     def write_chunk(self, position, data):
         size = getsize(self._path)
         if size < position + len(data):
-            with open(self._path, 'w') as f:
-                f.write('\0' * (position + len(data) - size))
-                f.seek(position)
-                f.write(data)
+            self._file.write('\0' * (position + len(data) - size))
+            self._file.seek(position)
+            self._file.write(data)
+            self._file.flush()
         else:
-            with open(self._path, 'w') as f:
-                f.seek(position)
-                f.write(data)
+            self._file.seek(position)
+            self._file.write(data)
+            self._file.flush()