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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 set_key(self, key):
 """ Set key policy condition. :param key: set key name. """

        is_non_empty_string(key)

        self.policies.append(('eq', '$key', key))
        self.form_data['key'] = key
        self.key = key