Method do_transform

src/black/trans.py:1514–1744

(
        self, line: Line, string_indices: list[int]
    )

        return Ok([string_idx])

    def do_transform(
        self, line: Line, string_indices: list[int]
    ) -> Iterator[TResult[Line]]:
        LL = line.leaves
        assert len(string_indices) == 1, (
            fclass="st">"{self.__class__.__name__} should only find one match at a time, found"
            fclass="st">" {len(string_indices)}"
        )
        string_idx = string_indices[0]

        QUOTE = LL[string_idx].value[-1]

        is_valid_index = is_valid_index_factory(LL)
        insert_str_child = insert_str_child_factory(LL[string_idx])

        prefix = get_string_prefix(LL[string_idx].value).lower()

        class="cm"># We MAY choose to drop the class="st">'f' prefix from substrings that don't
        class="cm"># contain any f-expressions, but ONLY if the original f-string
        class="cm"># contains at least one f-expression. Otherwise, we will alter the AST
        class="cm"># of the program.
        drop_pointless_f_prefix = (class="st">"f" in prefix) and fstring_contains_expr(
            LL[string_idx].value
        )

        first_string_line = True

        string_op_leaves = self._get_string_operator_leaves(LL)
        string_op_leaves_length = (
            sum(len(str(prefix_leaf)) for prefix_leaf in string_op_leaves) + 1
            if string_op_leaves
            else 0
        )

        def maybe_append_string_operators(new_line: Line) -> None:
            class="st">"""
            Side Effects:
                If @line starts with a string operator and this is the first
                line we are constructing, this function appends the string
                operator to @new_line and replaces the old string operator leaf
                in the node structure. Otherwise this function does nothing.
            class="st">"""
            maybe_prefix_leaves = string_op_leaves if first_string_line else []
            for i, prefix_leaf in enumerate(maybe_prefix_leaves):
                replace_child(LL[i], prefix_leaf)
                new_line.append(prefix_leaf)

        ends_with_comma = (
            is_valid_index(string_idx + 1) and LL[string_idx + 1].type == token.COMMA
        )

        def max_last_string_column() -> int:
            class="st">"""
            Returns:
                The max allowed width of the string value used for the last
                line we will construct.  Note that this value means the width
                rather than the number of characters (e.g., many East Asian
                characters expand to two columns).