How to use python decorators for session validation

I have some session class like

class Session:

def __init__(self,):
    self._token_response_dict = self.get_token_details(Session._token_payload, Session._token_headers)
    self.token = self._token_response_dict['access_token']
    self.token_valid_time = datetime.datetime.now() + datetime.timedelta(
        seconds=self._token_response_dict['expires_in'])
    self.token_type = self._token_response_dict['token_type']

def is_valid(self):
    return True if datetime.datetime.now() < self.token_valid_time else False

@staticmethod
def get_token_details(payload, headers):
    r = requests.post('http://webservice/token',
                      data=payload, headers=headers)
    return json.loads(r.json())

Before calling some function I have to check whether the session is valid or not. i am using session as a global variable like below.

def get_all_docs(client_id):

global session
if session.is_valid():
    headers = {
        'content-type': "application/json",
        'cache-control': "no-cache",
        'postman-token': "aff6cf56-dc63-ae16-762f-4637762733ce",
        'authorization': session.token_type + ' ' + session.token

    }
    payload = {
        'LastRefreshTime': datetime.datetime.now().strptime(os.environ['LastRefreshTime'], '%d-%m-%Y %H:%M:%S'),
        'ClientID': client_id
    }

    r = requests.get(Details.get_docs_url, params=payload, headers=headers)
    return json.loads(r.json())
else:
    session = Session()
    get_all_docs(client_id)

How to perform this validation using python decorators?