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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 format(self, template=None):
 """ Substitutes variables within template with that of fields' """

        pattern = r"(?:<([^<]*?)\$(\w+)([^>]*?)>)"
        s = sub(pattern, self._format_repl, template or self.template)
        s = self._str_fix_whitespace(s)
        return s