How to use the pycobertura.filesystem.DirectoryFileSystem function in pycobertura

def test_filesystem_directory__file_not_found():
    from pycobertura.filesystem import DirectoryFileSystem

    fs = DirectoryFileSystem('foo/bar/baz')

    expected_filepaths = {
        'Main.java': 'foo/bar/baz/Main.java',
        'search/BinarySearch.java': 'foo/bar/baz/search/BinarySearch.java',
        'search/ISortedArraySearch.java': 'foo/bar/baz/search/ISortedArraySearch.java',
        'search/LinearSearch.java': 'foo/bar/baz/search/LinearSearch.java',
    }

    for filename in expected_filepaths:
        try:
            with fs.open(filename) as f:
                pass
        except DirectoryFileSystem.FileNotFound as fnf:
            assert fnf.path == expected_filepaths[filename]

def test_filesystem_directory__with_source_prefix():
    from pycobertura.filesystem import DirectoryFileSystem

    fs = DirectoryFileSystem(
        'tests/',
        source_prefix='dummy'  # should resolve to tests/dummy/
    )

    expected_filepaths = {
        'dummy/dummy.py': 'dummy/dummy/dummy.py',
    }

    for filename in expected_filepaths:
        with fs.open(filename) as f:
            assert hasattr(f, 'read')

def test_filesystem_directory__returns_fileobject():
    from pycobertura.filesystem import DirectoryFileSystem

    fs = DirectoryFileSystem('tests/dummy')

    expected_filepaths = {
        'dummy/dummy.py': 'dummy/dummy/dummy.py',
    }

    for filename in expected_filepaths:
        with fs.open(filename) as f:
            assert hasattr(f, 'read')

from pycobertura.filesystem import DirectoryFileSystem

    fs = DirectoryFileSystem('foo/bar/baz')

    expected_filepaths = {
        'Main.java': 'foo/bar/baz/Main.java',
        'search/BinarySearch.java': 'foo/bar/baz/search/BinarySearch.java',
        'search/ISortedArraySearch.java': 'foo/bar/baz/search/ISortedArraySearch.java',
        'search/LinearSearch.java': 'foo/bar/baz/search/LinearSearch.java',
    }

    for filename in expected_filepaths:
        try:
            with fs.open(filename) as f:
                pass
        except DirectoryFileSystem.FileNotFound as fnf:
            assert fnf.path == expected_filepaths[filename]

The optional argument `ref` will be taken into account when
    instantiating a GitFileSystem, and it shall be a branch name, a commit
    ID or a git ref ID.
    """
    if source is None:
        if isinstance(report, str):
            # get the directory in which the coverage file lives
            source = os.path.dirname(report)

    if zipfile.is_zipfile(source):
        return ZipFileSystem(source, source_prefix=source_prefix)

    if ref:
        return GitFileSystem(source, ref)

    return DirectoryFileSystem(source, source_prefix=source_prefix)