def _normalize_f_string(self, string: str, prefix: str) -> str:
        """
        Pre-Conditions:
            * assert_is_leaf_string(@string)

        Returns:
            * If @string is an f-string that contains no f-expressions, we
            return a string identical to @string except that the 'f' prefix
            has been stripped and all double braces (i.e. '{{' or '}}') have
            been normalized (i.e. turned into '{' or '}').
                OR
            * Otherwise, we return @string.
        """
        assert_is_leaf_string(string)

        if "f" in prefix and not fstring_contains_expr(string):
            new_prefix = prefix.replace("f", "")

            temp = string[len(prefix) :]
            temp = re.sub(r"\{\{", "{", temp)
            temp = re.sub(r"\}\}", "}", temp)
            new_string = temp

            return f"{new_prefix}{new_string}"
        else:
            return string

        Pre-Conditions:
            * assert_is_leaf_string(@string)

        Returns:
            * If @string is an f-string that contains no f-expressions, we
            return a string identical to @string except that the 'f' prefix
            has been stripped and all double braces (i.e. '{{' or '}}') have
            been normalized (i.e. turned into '{' or '}').
                OR
            * Otherwise, we return @string.
        
src/black/trans.py