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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 derive_readonly(self):
 """ Figures out what fields should be readonly. We iterate our field_config to find all that have a readonly of true """

        readonly = list(self.readonly)
        for key, value in self.field_config.items():
            if 'readonly' in value and value['readonly']:
                readonly.append(key)

        return readonly