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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 getsize(self, key=None):
 """ Return the size in byte of the output associated to the given key. If no key is given, returns the total size of all files. """

        if key is None:
            return os.path.getsize(self.filename)
        return hdf5.ByteCounter.get_nbytes(
            h5py.File.__getitem__(self.hdf5, key))