How to use the graphviz.backend.view function in graphviz

def test_view_unknown_platform(unknown_platform):
    with pytest.raises(RuntimeError, match=r'platform'):
        view('nonfilepath')

def test_view(mocker, py2, platform, Popen, startfile, quiet):  # noqa: N803
    if quiet and py2:
        open_ = mocker.patch('__builtin__.open', mocker.mock_open())

    assert view('nonfilepath', quiet=quiet) is None

    if platform == 'windows':
        startfile.assert_called_once_with('nonfilepath')
        return

    if quiet and py2:
        open_.assert_called_once_with(os.devnull, 'w')
        kwargs = {'stderr': open_.return_value}
    elif quiet:
        kwargs = {'stderr': subprocess.DEVNULL}
    else:
        kwargs = {}

    if platform == 'darwin':
        Popen.assert_called_once_with(['open', 'nonfilepath'], **kwargs)
    elif platform in ('linux', 'freebsd'):

"""Start the right viewer based on file format and platform."""
        methodnames = [
            '_view_%s_%s' % (format, backend.PLATFORM),
            '_view_%s' % backend.PLATFORM,
        ]
        for name in methodnames:
            view_method = getattr(self, name, None)
            if view_method is not None:
                break
        else:
            raise RuntimeError('%r has no built-in viewer support for %r'
                               ' on %r platform' % (self.__class__, format,
                                                    backend.PLATFORM))
        view_method(filepath, quiet)

    _view_darwin = staticmethod(backend.view.darwin)
    _view_freebsd = staticmethod(backend.view.freebsd)
    _view_linux = staticmethod(backend.view.linux)
    _view_windows = staticmethod(backend.view.windows)


class Source(File):
    """Verbatim DOT source code string to be rendered by Graphviz.

    Args:
        source: The verbatim DOT source code string.
        filename: Filename for saving the source (defaults to ``'Source.gv'``).
        directory: (Sub)directory for source saving and rendering.
        format: Rendering output format (``'pdf'``, ``'png'``, ...).
        engine: Layout command used (``'dot'``, ``'neato'``, ...).
        encoding: Encoding for saving the source.

'_view_%s_%s' % (format, backend.PLATFORM),
            '_view_%s' % backend.PLATFORM,
        ]
        for name in methodnames:
            view_method = getattr(self, name, None)
            if view_method is not None:
                break
        else:
            raise RuntimeError('%r has no built-in viewer support for %r'
                               ' on %r platform' % (self.__class__, format,
                                                    backend.PLATFORM))
        view_method(filepath, quiet)

    _view_darwin = staticmethod(backend.view.darwin)
    _view_freebsd = staticmethod(backend.view.freebsd)
    _view_linux = staticmethod(backend.view.linux)
    _view_windows = staticmethod(backend.view.windows)


class Source(File):
    """Verbatim DOT source code string to be rendered by Graphviz.

    Args:
        source: The verbatim DOT source code string.
        filename: Filename for saving the source (defaults to ``'Source.gv'``).
        directory: (Sub)directory for source saving and rendering.
        format: Rendering output format (``'pdf'``, ``'png'``, ...).
        engine: Layout command used (``'dot'``, ``'neato'``, ...).
        encoding: Encoding for saving the source.

    Note:
        All parameters except ``source`` are optional. All of them can be changed

'_view_%s' % backend.PLATFORM,
        ]
        for name in methodnames:
            view_method = getattr(self, name, None)
            if view_method is not None:
                break
        else:
            raise RuntimeError('%r has no built-in viewer support for %r'
                               ' on %r platform' % (self.__class__, format,
                                                    backend.PLATFORM))
        view_method(filepath, quiet)

    _view_darwin = staticmethod(backend.view.darwin)
    _view_freebsd = staticmethod(backend.view.freebsd)
    _view_linux = staticmethod(backend.view.linux)
    _view_windows = staticmethod(backend.view.windows)


class Source(File):
    """Verbatim DOT source code string to be rendered by Graphviz.

    Args:
        source: The verbatim DOT source code string.
        filename: Filename for saving the source (defaults to ``'Source.gv'``).
        directory: (Sub)directory for source saving and rendering.
        format: Rendering output format (``'pdf'``, ``'png'``, ...).
        engine: Layout command used (``'dot'``, ``'neato'``, ...).
        encoding: Encoding for saving the source.

    Note:
        All parameters except ``source`` are optional. All of them can be changed
        under their corresponding attribute name after instance creation.

methodnames = [
            '_view_%s_%s' % (format, backend.PLATFORM),
            '_view_%s' % backend.PLATFORM,
        ]
        for name in methodnames:
            view_method = getattr(self, name, None)
            if view_method is not None:
                break
        else:
            raise RuntimeError('%r has no built-in viewer support for %r'
                               ' on %r platform' % (self.__class__, format,
                                                    backend.PLATFORM))
        view_method(filepath, quiet)

    _view_darwin = staticmethod(backend.view.darwin)
    _view_freebsd = staticmethod(backend.view.freebsd)
    _view_linux = staticmethod(backend.view.linux)
    _view_windows = staticmethod(backend.view.windows)


class Source(File):
    """Verbatim DOT source code string to be rendered by Graphviz.

    Args:
        source: The verbatim DOT source code string.
        filename: Filename for saving the source (defaults to ``'Source.gv'``).
        directory: (Sub)directory for source saving and rendering.
        format: Rendering output format (``'pdf'``, ``'png'``, ...).
        engine: Layout command used (``'dot'``, ``'neato'``, ...).
        encoding: Encoding for saving the source.

    Note: