Как можно изменить синтаксис?

    def get_text_coord (s: str, i: int):
        for row_number, line in enumerate (s.splitlines(keepends = True), 1):
            if i < len(line): return f'{row_number}.{i}'
            i -= len(line)
    for tag in textbox.tag_names (): textbox.tag_remove (tag, 1.0, 'end')
    s = textbox.get (1.0, 'end')
    tokens = lexer.get_tokens_unprocessed(s)
    for i , token_type, token in tokens:
        j = i + len (token)
        if token_type in token_type_to_tag: textbox.tag_add (token_type_to_tag [token_type], get_text_coord (s, i), get_text_coord (s, j))
    textbox.edit_modified (0)

Python 3.4.3 жалуется на неправильный синтаксис:

Traceback (most recent call last):
  File "coddy.pyw", line 46, in <module>
    import coddy_syntax as syntax
  File "src\coddy_syntax.py", line 52
    if i < len(line): return f'{row_number}.{i}'
                                               ^
SyntaxError: invalid syntax

Как можно изменить синтаксис?