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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 xpathEvalExpression(self, str):
 """Evaluate the XPath expression in the given context. """

        ret = libxml2mod.xmlXPathEvalExpression(str, self._o)
        if ret is None:raise xpathError('xmlXPathEvalExpression() failed')
        return xpathObjectRet(ret)