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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_obj, variables=None):
 """ Constructs an Objective from the provided json-object. Example -------- """

        if variables is None:
            variables = {}
        expression = parse_expr(json_obj["expression"], variables)
        return cls(
            expression,
            direction=json_obj["direction"],
            name=json_obj["name"]
        )