<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _validate_function(self, func, name):
 """ Tests `self.style_function` and `self.highlight_function` to ensure they are functions returning dictionaries. """

        test_feature = self.data['features'][0]
        if not callable(func) or not isinstance(func(test_feature), dict):
            raise ValueError('{} should be a function that accepts items from '
                             'data[\'features\'] and returns a dictionary.'
                             .format(name))