Python 语法类 __repr__ 方法中 fstring 上的错误 [duplicate]

问：

我的代码：

class Book:
    all = []
    def __init__(self, ISBN, title, subject, publisher, language, number_of_pages):
        self.__ISBN = ISBN
        self.__title = title
        self.__subject = subject
        self.__publisher = publisher
        self.__language = language
        self.__number_of_pages = number_of_pages
        self.__authors = []

        Book.all.append(self)

    def __repr__(self):
        return f"Title {self.__title} Subject {self.__subject}"

b = Book(23498723,"this","fiction","penguin","en",196)

错误：

    return f"Title {self.__title} Subject {self.__subject}"
                                                          ^
SyntaxError: invalid syntax

谁能解释为什么这是一个错误？