def format_str(
    src_contents: str, *, mode: Mode, lines: Collection[tuple[int, int]] = ()
) -> str:
    """Reformat a string and return new contents.

    `mode` determines formatting options, such as how many characters per line are
    allowed.  Example:

    >>> import black
    >>> print(black.format_str("def f(arg:str='')->None:...", mode=black.Mode()))
    def f(arg: str = "") -> None:
        ...

    A more complex example:

    >>> print(
    ...   black.format_str(
    ...     "def f(arg:str='')->None: hey",
    ...     mode=black.Mode(
    ...       target_versions={black.TargetVersion.PY36},
    ...       line_length=10,
    ...       string_normalization=False,
    ...       is_pyi=False,
    ...     ),
    ...   ),
    ... )
    def f(
        arg: str = '',
    ) -> None:
        hey

    """
    if lines:
        lines = sanitized_lines(lines, src_contents)
        if not lines:
            return src_contents  # Nothing to format
    dst_contents = _format_str_once(src_contents, mode=mode, lines=lines)
    # Forced second pass to work around optional trailing commas (becoming
    # forced trailing commas on pass 2) interacting differently with optional
    # parentheses.  Admittedly ugly.
    if src_contents != dst_contents:
        if lines:
            lines = adjusted_lines(lines, src_contents, dst_contents)
        return _format_str_once(dst_contents, mode=mode, lines=lines)
    return dst_contents
Reformat a string and return new contents.

    `mode` determines formatting options, such as how many characters per line are
    allowed.  Example:

    >>> import black
    >>> print(black.format_str("def f(arg:str='')->None:...", mode=black.Mode()))
    def f(arg: str = "") -> None:
        ...

    A more complex example:

    >>> print(
    ...   black.format_str(
    ...     "def f(arg:str='')->None: hey",
    ...     mode=black.Mode(
    ...       target_versions={black.TargetVersion.PY36},
    ...       line_length=10,
    ...       string_normalization=False,
    ...       is_pyi=False,
    ...     ),
    ...   ),
    ... )
    def f(
        arg: str = '',
    ) -> None:
        hey

    
src/black/__init__.py