How to use the coverage.misc.contract function in coverage

        @contract(text='bytes|None')
        def need_bytes(text=None):
            return text

    @contract(returns='unicode')
    def unicode_filename(filename):
        """Return a Unicode version of `filename`."""
        if isinstance(filename, str):
            encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
            filename = filename.decode(encoding, "replace")
        return filename
else:

@contract(returns='unicode')
def relative_filename(filename):
    """Return the relative form of `filename`.

    The file name will be relative to the current directory when the
    `set_relative_directory` was called.

    """
    fnorm = os.path.normcase(filename)
    if fnorm.startswith(RELATIVE_DIR):
        filename = filename[len(RELATIVE_DIR):]
    return unicode_filename(filename)

    @contract(returns='ArcStarts')
    def _handle__TryExcept(self, node):
        # Python 2.7 uses separate TryExcept and TryFinally nodes. If we get
        # TryExcept, it means there was no finally, so fake it, and treat as
        # a general Try node.
        node.finalbody = []
        return self._handle__Try(node)

    @contract(returns=int)
    def line_for_node(self, node):
        """What is the right line number to use for this node?

        This dispatches to _line__Node functions where needed.

        """
        node_name = node.__class__.__name__
        handler = getattr(self, "_line__" + node_name, None)
        if handler is not None:
            return handler(node)
        else:
            return node.lineno

    @contract(returns='ArcStarts')
    def _handle__If(self, node):
        start = self.line_for_node(node.test)
        from_start = ArcStart(start, cause="the condition on line {lineno} was never true")
        exits = self.add_body_arcs(node.body, from_start=from_start)
        from_start = ArcStart(start, cause="the condition on line {lineno} was never false")
        exits |= self.add_body_arcs(node.orelse, from_start=from_start)
        return exits

    @contract(returns='unicode')
    def source(self):
        if self._source is None:
            self._source = get_python_source(self.filename)
        return self._source

@contract(numbits='blob', returns='list[int]')
def numbits_to_nums(numbits):
    """Convert a numbits into a list of numbers.

    Arguments:
        numbits: a binary blob, the packed number set.

    Returns:
        A list of ints.

    When registered as a SQLite function by :func:`register_sqlite_functions`,
    this returns a string, a JSON-encoded list of ints.

    """
    nums = []
    for byte_i, byte in enumerate(bytes_to_ints(numbits)):
        for bit_i in range(8):

    @contract(filename='unicode', returns='unicode')
    def unicode_filename(filename):
        """Return a Unicode version of `filename`."""
        return filename

@contract(returns='unicode')
def canonical_filename(filename):
    """Return a canonical file name for `filename`.

    An absolute path with no redundant components and normalized case.

    """
    if filename not in CANONICAL_FILENAME_CACHE:
        cf = filename
        if not os.path.isabs(filename):
            for path in [os.curdir] + sys.path:
                if path is None:
                    continue
                f = os.path.join(path, filename)
                try:
                    exists = os.path.exists(f)
                except UnicodeError: