Return a dictionary representing the environment variables used to
        set the proxy settings.

def get_environment(self):
        '''
        Return a dictionary representing the environment variables used to
        set the proxy settings.
        '''
        env = {}
        if self.http:
            env['http_proxy'] = env['HTTP_PROXY'] = self.http
        if self.https:
            env['https_proxy'] = env['HTTPS_PROXY'] = self.https
        if self.ftp:
            env['ftp_proxy'] = env['FTP_PROXY'] = self.ftp
        if self.no_proxy:
            env['no_proxy'] = env['NO_PROXY'] = self.no_proxy
        return env