Preserve Annotated dependency metadata for stringified refs

fastapi/dependencies/utils.py

@@ -81,6 +81,13 @@ multipart_not_installed_error = (
     'You can install "python-multipart" with: \n\n'
     "pip install python-multipart\n"
 )
+
+
+class _ForwardRefNamespace(dict[str, Any]):
+    def __missing__(self, key: str) -> Any:
+        value = ForwardRef(key)
+        self[key] = value
+        return value
 multipart_incorrect_install_error = (
     'Form data requires "python-multipart" to be installed. '
     'It seems you installed "multipart" instead. \n'
@@ -244,8 +251,13 @@ def get_typed_signature(call: Callable[..., Any]) -> inspect.Signature:
 
 def get_typed_annotation(annotation: Any, globalns: dict[str, Any]) -> Any:
     if isinstance(annotation, str):
-        annotation = ForwardRef(annotation)
-        annotation = evaluate_forwardref(annotation, globalns, globalns)  # ty: ignore[deprecated]
+        try:
+            annotation = eval(annotation, globalns, globalns)
+        except NameError:
+            # Preserve the outer typing structure (e.g. Annotated[..., Depends(...)])
+            # even when some inner symbols are not defined yet.
+            forwardref_globalns = _ForwardRefNamespace(globalns)
+            annotation = eval(annotation, forwardref_globalns, forwardref_globalns)
         if annotation is type(None):
             return None
     return annotation

tests/test_stringified_annotations_forwardrefs.py

@@ -0,0 +1,79 @@
+from __future__ import annotations
+
+from fastapi.testclient import TestClient
+
+from .utils import needs_py310
+
+
+def _build_app(source: str):
+    namespace: dict[str, object] = {}
+    exec(source, namespace, namespace)
+    return namespace["app"]
+
+
+@needs_py310
+def test_late_defined_annotated_dependency_forwardref_is_not_treated_as_query_param():
+    app = _build_app("""
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Annotated
+
+from fastapi import Depends, FastAPI
+
+app = FastAPI()
+
+def get_potato() -> Potato:
+    return Potato(color="red", size=10)
+
+@app.get("/")
+async def read_root(potato: Annotated[Potato, Depends(get_potato)]):
+    return {"color": potato.color, "size": potato.size}
+
+@dataclass
+class Potato:
+    color: str
+    size: int
+""")
+    client = TestClient(app)
+
+    response = client.get("/")
+    assert response.status_code == 200, response.text
+    assert response.json() == {"color": "red", "size": 10}
+
+    operation = client.get("/openapi.json").json()["paths"]["/"]["get"]
+    assert "parameters" not in operation
+
+
+@needs_py310
+def test_stringified_annotated_dependency_with_defined_type_remains_a_dependency():
+    app = _build_app("""
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Annotated
+
+from fastapi import Depends, FastAPI
+
+app = FastAPI()
+
+@dataclass
+class Potato:
+    color: str
+    size: int
+
+def get_potato() -> Potato:
+    return Potato(color="gold", size=7)
+
+@app.get("/")
+async def read_root(potato: Annotated["Potato", Depends(get_potato)]):
+    return {"color": potato.color, "size": potato.size}
+""")
+    client = TestClient(app)
+
+    response = client.get("/")
+    assert response.status_code == 200, response.text
+    assert response.json() == {"color": "gold", "size": 7}
+
+    operation = client.get("/openapi.json").json()["paths"]["/"]["get"]
+    assert "parameters" not in operation