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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 _format_target(self, net, f, idx):
 """Apply formatting to the target filename template."""

        if f is None:
            return None
        if isinstance(f, str):
            f = self.fn_prefix + f.format(
                net=net,
                last_epoch=net.history[idx],
                last_batch=net.history[idx, 'batches', -1],
            )
            return os.path.join(self.dirname, f)
        return f