GH-36: Memorize access_token property

Author: ArtyomVancyan

src/fastapi_oauth2/core.py

@@ -54,6 +54,7 @@ class OAuth2Core:
     _oauth_client: Optional[WebApplicationClient] = None
     _authorization_endpoint: str = None
     _token_endpoint: str = None
+    _access_token: str = None
     _state: str = None
 
     def __init__(self, client: OAuth2Client) -> None:
@@ -70,7 +71,9 @@ def __init__(self, client: OAuth2Client) -> None:
 
     @property
     def access_token(self) -> str:
-        return self._oauth_client.access_token
+        if not self._access_token:
+            self._access_token = self._oauth_client.access_token
+        return self._access_token
 
     def get_redirect_uri(self, request: Request) -> str:
         return urljoin(str(request.base_url), "/oauth2/%s/token" % self.provider)