Method do_transform

src/black/trans.py:2227–2348

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

        return None

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

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

        comma_idx = -1
        ends_with_comma = False
        if LL[comma_idx].type == token.COMMA:
            ends_with_comma = True

        leaves_to_steal_comments_from = [LL[string_idx]]
        if ends_with_comma:
            leaves_to_steal_comments_from.append(LL[comma_idx])

        # --- First Line
        first_line = line.clone()
        left_leaves = LL[:string_idx]

        # We have to remember to account for (possibly invisible) LPAR and RPAR
        # leaves that already wrapped the target string. If these leaves do
        # exist, we will replace them with our own LPAR and RPAR leaves.
        old_parens_exist = False
        if left_leaves and left_leaves[-1].type == token.LPAR:
            old_parens_exist = True
            leaves_to_steal_comments_from.append(left_leaves[-1])
            left_leaves.pop()

        append_leaves(first_line, line, left_leaves)

        lpar_leaf = Leaf(token.LPAR, "(")
        if old_parens_exist:
            replace_child(LL[string_idx - 1], lpar_leaf)
        else:
            insert_str_child(lpar_leaf)
        first_line.append(lpar_leaf)

        # We throw inline comments that were originally to the right of the
        # target string to the top line. They will now be shown to the right of
        # the LPAR.
        for leaf in leaves_to_steal_comments_from:
            for comment_leaf in line.comments_after(leaf):
                first_line.append(comment_leaf, preformatted=True)

        yield Ok(first_line)

        # --- Middle (String) Line
        # We only need to yield one (possibly too long) string line, since the
        # `StringSplitter` will break it down further if necessary.
        string_value = LL[string_idx].value
        string_line = Line(