'Fetch an access token.
This is the final step in the OAuth 1 workflow. An access token is
obtained using all previously obtained credentials, including the
verifier from the authorization step.
Note that a previously set verifier will be reset for your
convenience, or else signature creation will be incorrect on
consecutive requests.
>>> access_token_url = \'https://api.twitter.com/oauth/access_token\'
>>> redirect_response = \'https://127.0.0.1/callback?oauth_token=kjerht2309uf&oauth_token_secret=lsdajfh923874&oauth_verifier=w34o8967345\'
>>> oauth_session = OAuth1Session(\'client-key\', client_secret=\'secret\')
>>> oauth_session.parse_authorization_response(redirect_response)
\'oauth_token: \'kjerht2309u\',
\'oauth_token_secret: \'lsdajfh923874\',
\'oauth_verifier: \'w34o8967345\',
>>> oauth_session.fetch_access_token(access_token_url)
\'oauth_token\': \'sdf0o9823sjdfsdf\',
\'oauth_token_secret\': \'2kjshdfp92i34asdasd\','

def fetch_access_token(self, url, verifier=None, **request_kwargs):

    if verifier:
        self._client.client.verifier = verifier
    if (not getattr(self._client.client, u'verifier', None)):
        raise VerifierMissing(u'No    client    verifier    has    been    set.')
    token = self._fetch_token(url, **request_kwargs)
    log.debug(u'Resetting    verifier    attribute,    should    not    be    used    anymore.')
    self._client.client.verifier = None
    return token
