Pass tempfile_suffix argument to PromptSession. This ensures that when the open_in_editor() method of the current buffer (a prompt_toolkit.buffer.Buffer) is called, the tempfile opened in the editor will always have a .py extension, to support editor syntax highlighting. The open_input_in_editor(event) function associated with the f2 keybinding manually sets the tempfile_suffix property of the current buffer before calling its open_in_editor() method. The vi keybinding (v) and emacs mode keybinding (c-x c-e) are handled by prompt_toolkit directly, bypassing this function, so the tempfile created when they are used is a .txt file. Passing the tempfile_suffix argument to the PrompSession constructor ensures that .py extension is always used.