<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def to_graphtool(self):
 r"""Export the graph to graph-tool. Edge weights are stored as an edge property map, under the name "weight". Signals are stored as vertex property maps, under their name in the :attr:`signals` dictionary. `N`-dimensional signals are broken into `N` 1-dimensional signals. They will eventually be joined back together on import. Returns ------- graph : :class:`graph_tool.Graph` A graph-tool graph object. See Also -------- to_networkx : export to NetworkX save : save to a file Examples -------- True 2.3 1.0 Another common goal is to use graph-tool to compute some properties to be imported back in the PyGSP as signals. """


        # See gt.value_types() for the list of accepted types.
        # See the definition of _type_alias() for a list of aliases.
        # Mapping from https://docs.scipy.org/doc/numpy/user/basics.types.html.
        convert = {
            np.bool_: 'bool',
            np.int8: 'int8_t',
            np.int16: 'int16_t',
            np.int32: 'int32_t',
            np.int64: 'int64_t',
            np.short: 'short',
            np.intc: 'int',
            np.uintc: 'unsigned int',
            np.long: 'long',
            np.longlong: 'long long',
            np.uint: 'unsigned long',
            np.single: 'float',
            np.double: 'double',
            np.longdouble: 'long double',
        }

        gt = _import_graphtool()
        graph = gt.Graph(directed=self.is_directed())

        sources, targets, weights = self.get_edge_list()
        graph.add_edge_list(np.asarray((sources, targets)).T)
        try:
            dtype = convert[weights.dtype.type]
        except KeyError:
            raise TypeError("Type {} of the edge weights is not supported."
                            .format(weights.dtype))
        prop = graph.new_edge_property(dtype)
        prop.get_array()[:] = weights
        graph.edge_properties['weight'] = prop

        self._break_signals()
        for name, signal in self.signals.items():
            try:
                dtype = convert[signal.dtype.type]
            except KeyError:
                raise TypeError("Type {} of signal {} is not supported."
                                .format(signal.dtype, name))
            prop = graph.new_vertex_property(dtype)
            prop.get_array()[:] = signal
            graph.vertex_properties[name] = prop

        return graph