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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 from_json(cls, json):
 """Instantiates an instance of this InputReader for the given shard spec."""

    return cls(json[cls.BLOB_KEY_PARAM],
               json[cls.INITIAL_POSITION_PARAM],
               json[cls.END_POSITION_PARAM])