How to use the black.Line function in black

src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
    dst_contents = []
    future_imports = get_future_imports(src_node)
    if mode.target_versions:
        versions = mode.target_versions
    else:
        versions = detect_target_versions(src_node)
    normalize_fmt_off(src_node)
    lines = LineGenerator(
        remove_u_prefix="unicode_literals" in future_imports
        or supports_feature(versions, Feature.UNICODE_LITERALS),
        is_pyi=mode.is_pyi,
        normalize_strings=mode.string_normalization,
    )
    elt = EmptyLineTracker(is_pyi=mode.is_pyi)
    empty_line = Line()
    after = 0
    split_line_features = {
        feature
        for feature in {Feature.TRAILING_COMMA_IN_CALL, Feature.TRAILING_COMMA_IN_DEF}
        if supports_feature(versions, feature)
    }
    for current_line in lines.visit(src_node):
        for _ in range(after):
            dst_contents.append(str(empty_line))
        before, after = elt.maybe_empty_lines(current_line)
        for _ in range(before):
            dst_contents.append(str(empty_line))
        for line in split_line(
            current_line, line_length=mode.line_length, features=split_line_features
        ):
            dst_contents.append(str(line))

def standalone_comment_split(
    line: Line, features: Collection[Feature] = ()
) -> Iterator[Line]:
    """Split standalone comments from the rest of the line."""
    if not line.contains_standalone_comments(0):
        raise CannotSplit("Line does not have any standalone comments")

    current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)

    def append_to_line(leaf: Leaf) -> Iterator[Line]:
        """Append `leaf` to current line or to new line if appending impossible."""
        nonlocal current_line
        try:
            current_line.append_safe(leaf, preformatted=True)
        except ValueError:
            yield current_line

            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
            current_line.append(leaf)

    for leaf in line.leaves:
        yield from append_to_line(leaf)

        for comment_after in line.comments_after(leaf):

def bracket_split_build_line(
    leaves: List[Leaf], original: Line, opening_bracket: Leaf, *, is_body: bool = False
) -> Line:
    """Return a new line with given `leaves` and respective comments from `original`.

    If `is_body` is True, the result line is one-indented inside brackets and as such
    has its first leaf's prefix normalized and a trailing comma added when expected.
    """
    result = Line(depth=original.depth)
    if is_body:
        result.inside_brackets = True
        result.depth += 1
        if leaves:
            # Since body is a new indent level, remove spurious leading whitespace.
            normalize_prefix(leaves[0], inside_brackets=True)
            # Ensure a trailing comma for imports, but be careful not to add one after
            # any comments.
            if original.is_import:
                for i in range(len(leaves) - 1, -1, -1):
                    if leaves[i].type == STANDALONE_COMMENT:
                        continue
                    elif leaves[i].type == token.COMMA:
                        break
                    else:
                        leaves.insert(i + 1, Leaf(token.COMMA, ","))

if current_line.depth and newlines:
            newlines -= 1
        return newlines, 0


@dataclass
class LineGenerator(Visitor[Line]):
    """Generates reformatted Line objects.  Empty lines are not emitted.

    Note: destroys the tree it's visiting by mutating prefixes of its leaves
    in ways that will no longer stringify to valid Python code on the tree.
    """

    is_pyi: bool = False
    normalize_strings: bool = True
    current_line: Line = Factory(Line)
    remove_u_prefix: bool = False

    def line(self, indent: int = 0) -> Iterator[Line]:
        """Generate a line.

        If the line is empty, only emit if it makes sense.
        If the line is too long, split it first and then generate.

        If any lines were generated, set up a new current_line.
        """
        if not self.current_line:
            self.current_line.depth += indent
            return  # Line is empty, don't emit. Creating a new one unnecessary.

        complete_line = self.current_line
        self.current_line = Line(depth=complete_line.depth + indent)

def append_to_line(leaf: Leaf) -> Iterator[Line]:
        """Append `leaf` to current line or to new line if appending impossible."""
        nonlocal current_line
        try:
            current_line.append_safe(leaf, preformatted=True)
        except ValueError:
            yield current_line

            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
            current_line.append(leaf)