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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 _exclude_files_parser(cls, key):
 """Returns a parser function to make sure field inputs are not files. Parses a value after getting the key so error messages are more informative. :param key: :rtype: callable """

        def parser(value):
            exclude_directive = 'file:'
            if value.startswith(exclude_directive):
                raise ValueError(
                    'Only strings are accepted for the {0} field, '
                    'files are not accepted'.format(key))
            return value
        return parser