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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def peak_load(self):
 """ Cumulative peak load capacity of generators of this grid Returns ------- float Ad-hoc calculated or cached peak load capacity """

        if self._peak_load is None:
            self._peak_load = sum(
                [_.peak_load.sum()
                 for _ in self.graph.nodes_by_attribute('load')])

        return self._peak_load