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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 _averageFromHeader(self, header, keyword):
 """ Averages out values taken from header. The keywords where to read values from are passed as a comma-separated list. """

        _list = ''
        for _kw in keyword.split(','):
            if _kw in header:
                _list = _list + ',' + str(header[_kw])
            else:
                return None
        return self._averageFromList(_list)