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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 _setfile(self, filename, length, offset):
 """Use file as source of bits."""

        source = open(filename, 'rb')
        if offset is None:
            offset = 0
        if length is None:
            length = os.path.getsize(source.name) * 8 - offset
        byteoffset, offset = divmod(offset, 8)
        bytelength = (length + byteoffset * 8 + offset + 7) // 8 - byteoffset
        m = MmapByteArray(source, bytelength, byteoffset)
        if length + byteoffset * 8 + offset > m.filelength * 8:
            raise CreationError("File is not long enough for specified "
                                "length and offset.")
        self._datastore = ConstByteStore(m, length, offset)