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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 site_name(self, site_name):
 """Function that sets and checks the site name and set url. Parameters: site_name (str):
 The site name in 'SITE_LIST', default sites. Raises: PybooruError: When 'site_name' isn't valid. """

        if site_name in SITE_LIST:
            self.__site_name = site_name
            self.__site_url = SITE_LIST[site_name]['url']
        else:
            raise PybooruError(
                "The 'site_name' is not valid, specify a valid 'site_name'.")