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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 _get_root_files(self, directory):
 """Retrieve files within the root directory"""

        if len(self.filepaths) is 0:
            if self.filters:
                root_files = [(directory, f) for f in os.listdir(directory)
                              if os.path.isfile(os.path.join(directory, f))
                              and self.filters.validate(f)
                              and self.filters.get_level(f) == self.filters.max_level]
            else:
                root_files = [(directory, f) for f in os.listdir(directory)
                              if os.path.isfile(os.path.join(directory, f))]
            self.add_path(root_files)