Implement endpoint alternatives (GH-49)

Co-authored-by: Tatayoyoh <r.dubodan@gmail.com>

Author: ArtyomVancyan

docs/integration/configuration.md

@@ -51,6 +51,28 @@ OAuth2Client(
 )
 ```
 
+## Backends
+
+If endpoints of a backend need to use `authorization_url` and `access_token_url`, then please ensure
+`AUTHORIZATION_URL` and `ACCESS_TOKEN_URL` are set to `None` or at least empty strings so they can be called
+alternatively.
+
+```python
+from social_core.backends import facebook
+
+
+class FacebookOAuth2(facebook.FacebookOAuth2):
+    AUTHORIZATION_URL = None
+    ACCESS_TOKEN_URL = None
+    USER_DATA_URL = "https://graph.facebook.com/v18.0/me"
+
+    def authorization_url(self):
+        return "https://www.facebook.com/v18.0/dialog/oauth"
+
+    def access_token_url(self):
+        return "https://graph.facebook.com/v18.0/oauth/access_token"
+```
+
 ## Claims
 
 The `Claims` class is used to define the claim mapping for a given OAuth2 client, and it has `display_name`, `identity`,

src/fastapi_oauth2/core.py

@@ -1,4 +1,5 @@
 import json
+import os
 import random
 import re
 import string
@@ -33,7 +34,10 @@ def absolute_uri(self, path=None) -> str:
         return path
 
     def get_setting(self, name) -> Any:
-        """Mocked setting method."""
+        value = os.getenv(name)
+        if value is None:
+            raise KeyError
+        return value
 
     @staticmethod
     def get_json(url, method='GET', *args, **kwargs) -> httpx.Response:
@@ -64,8 +68,8 @@ def __init__(self, client: OAuth2Client) -> None:
         self.provider = client.backend.name
         self.redirect_uri = client.redirect_uri
         self.backend = client.backend(OAuth2Strategy())
-        self._authorization_endpoint = client.backend.AUTHORIZATION_URL
-        self._token_endpoint = client.backend.ACCESS_TOKEN_URL
+        self._authorization_endpoint = client.backend.AUTHORIZATION_URL or self.backend.authorization_url()
+        self._token_endpoint = client.backend.ACCESS_TOKEN_URL or self.backend.access_token_url()
         self._oauth_client = WebApplicationClient(self.client_id)
 
     @property

tests/test_backends.py

@@ -1,3 +1,5 @@
+import os
+
 import pytest
 
 from fastapi_oauth2.client import OAuth2Client
@@ -6,6 +8,9 @@
 
 @pytest.mark.anyio
 async def test_core_init_with_all_backends(backends):
+    os.environ["OIDC_ENDPOINT"] = "https://oidctest.wsweet.org"
+    os.environ["BASE_URL"] = "https://oidctest.wsweet.org"
+
     for backend in backends:
         try:
             OAuth2Core(OAuth2Client(