Fix: Convert scopes set to list in Credentials.__init__

This change ensures that if a set of scopes is passed to Credentials, it is converted to a list. This prevents issues with JSON serialization (to_json failure) and ensures consistent mutability behavior for the scopes property.

Fixes #1145

Author: google-labs-jules[bot]

google/oauth2/credentials.py

@@ -141,7 +141,10 @@ def __init__(
         self.expiry = expiry
         self._refresh_token = refresh_token
         self._id_token = id_token
-        self._scopes = scopes
+        if scopes is not None and isinstance(scopes, set):
+            self._scopes = list(scopes)
+        else:
+            self._scopes = scopes
         self._default_scopes = default_scopes
         self._granted_scopes = granted_scopes
         self._token_uri = token_uri
@@ -207,7 +210,7 @@ def refresh_token(self):
 
     @property
     def scopes(self):
-        """Optional[str]: The OAuth 2.0 permission scopes."""
+        """Optional[Sequence[str]]: The OAuth 2.0 permission scopes."""
         return self._scopes
 
     @property

tests/oauth2/test_credentials.py

@@ -55,6 +55,31 @@ def make_credentials(cls):
             enable_reauth_refresh=True,
         )
 
+    def test_init_with_set_scopes(self):
+        # Regression test for https://github.com/googleapis/google-auth-library-python/issues/1145
+        scopes_set = {"a", "b"}
+        creds = credentials.Credentials(token="token", scopes=scopes_set)
+
+        # Verify scopes are converted to a list
+        assert isinstance(creds.scopes, list)
+        assert set(creds.scopes) == scopes_set
+
+        # Verify internal storage is a list (ensure consistent mutability)
+        assert isinstance(creds._scopes, list)
+
+        # Verify consistency (property returns the same object)
+        assert creds.scopes is creds.scopes
+
+        # Verify modifications persist
+        creds.scopes.append("c")
+        assert "c" in creds.scopes
+        assert len(creds.scopes) == 3
+
+        # Verify to_json works
+        json_output = creds.to_json()
+        data = json.loads(json_output)
+        assert set(data["scopes"]) == {"a", "b", "c"}
+
     def test_default_state(self):
         credentials = self.make_credentials()
         assert not credentials.valid