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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_authenticity_token(self, url=_SIGNIN_URL):
 """ Returns an authenticity_token, mandatory for signing in """

        res = self.client._get(url=url, expected_status_code=200)
        soup = BeautifulSoup(res.text, _DEFAULT_BEAUTIFULSOUP_PARSER)
        selection = soup.select(_AUTHENTICITY_TOKEN_SELECTOR)
        try:
            authenticity_token = selection[0].get("content")
        except:
            raise ValueError(
                "authenticity_token not found in {} with {}\n{}".format(
                 _SIGNIN_URL, _AUTHENTICITY_TOKEN_SELECTOR, res.text))
        return authenticity_token