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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_proxy(self, proxystr=''):
 """ Get the proxy given the option passed on the command line. If an empty string is passed it looks at the HTTP_PROXY environment variable. """

        if not proxystr:
            proxystr = os.environ.get('HTTP_PROXY', '')
        if proxystr:
            if '@' in proxystr:
                user_password, server_port = proxystr.split('@', 1)
                if ':' in user_password:
                    user, password = user_password.split(':', 1)
                else:
                    user = user_password
                    prompt = 'Password for %s@%s: ' % (user, server_port)
                    password = urllib.quote(getpass.getpass(prompt))
                return '%s:%s@%s' % (user, password, server_port)
            else:
                return proxystr
        else:
            return None