def auto_newline(buffer: Buffer) -> None:
    r"""
    Insert \n at the cursor position. Also add necessary padding.
    """
    insert_text = buffer.insert_text

    if buffer.document.current_line_after_cursor:
        # When we are in the middle of a line. Always insert a newline.
        insert_text("\n")
    else:
        # Go to new line, but also add indentation.
        current_line = buffer.document.current_line_before_cursor.rstrip()
        insert_text("\n")

        # Unident if the last line ends with 'pass', remove four spaces.
        unindent = current_line.rstrip().endswith(" pass")

        # Copy whitespace from current line
        current_line2 = current_line[4:] if unindent else current_line

        for c in current_line2:
            if c.isspace():
                insert_text(c)
            else:
                break

        # If the last line ends with a colon, add four extra spaces.
        if current_line[-1:] == ":":
            for x in range(4):
                insert_text(" ")

    Insert \n at the cursor position. Also add necessary padding.
    
src/ptpython/key_bindings.py