Update routing.py to handle circular references

fastapi/routing.py

@@ -1204,6 +1204,7 @@ def read_users():
  app.include_router(internal_router)
  ```
  """
+ assert self is not router, "Router cannot be the same as parent"
  if prefix:
  assert prefix.startswith("/"), "A path prefix must start with '/'"
  assert not prefix.endswith(

tests/test_router_circular_import.py

@@ -0,0 +1,11 @@
+import pytest
+from fastapi import APIRouter, FastAPI
+
+
+def test_router_circular_import():
+ app = FastAPI()
+ router = APIRouter()
+
+ app.include_router(router)
+ with pytest.raises(AssertionError, match="Router cannot be the same as parent"):
+ router.include_router(router)