🔧 Add throwing error on unresolvable annotations and add comprehensive tests

fastapi/_compat/__init__.py

@@ -26,7 +26,7 @@ from .v2 import Undefined as Undefined
 from .v2 import Url as Url
 from .v2 import copy_field_info as copy_field_info
 from .v2 import create_body_model as create_body_model
-from .v2 import evaluate_forwardref as evaluate_forwardref  # ty: ignore[deprecated]
+from .v2 import evaluate_forwardref as evaluate_forwardref
 from .v2 import get_cached_model_fields as get_cached_model_fields
 from .v2 import get_definitions as get_definitions
 from .v2 import get_flat_models_from_fields as get_flat_models_from_fields

fastapi/_compat/v2.py

@@ -26,6 +26,7 @@ from pydantic._internal._schema_generation_shared import (  # type: ignore[attr-
     GetJsonSchemaHandler as GetJsonSchemaHandler,
 )
 from pydantic._internal._typing_extra import eval_type_lenient  # ty: ignore[deprecated]
+from pydantic._internal._typing_extra import try_eval_type  # ty: ignore[deprecated]
 from pydantic.fields import FieldInfo as FieldInfo
 from pydantic.json_schema import GenerateJsonSchema as _GenerateJsonSchema
 from pydantic.json_schema import JsonSchemaValue as JsonSchemaValue
@@ -38,7 +39,7 @@ from pydantic_core.core_schema import (
 
 RequiredParam = PydanticUndefined
 Undefined = PydanticUndefined
-evaluate_forwardref = eval_type_lenient  # ty: ignore[deprecated]
+evaluate_forwardref = try_eval_type
 
 
 class GenerateJsonSchema(_GenerateJsonSchema):

fastapi/dependencies/utils.py

@@ -33,7 +33,7 @@ from fastapi._compat import (
     Undefined,
     copy_field_info,
     create_body_model,
-    evaluate_forwardref,  # ty: ignore[deprecated]
+    evaluate_forwardref,
     field_annotation_is_scalar,
     field_annotation_is_scalar_sequence,
     field_annotation_is_sequence,
@@ -245,7 +245,17 @@ 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]
+    if isinstance(annotation, ForwardRef):
+        annotation, is_evaluated = evaluate_forwardref(annotation, globalns, globalns)  # ty: ignore[deprecated]
+        if not is_evaluated:
+            raise NameError(
+                f"Could not resolve annotation {annotation.__forward_arg__!r}. "
+                f"Make sure the type is defined or imported before it is used "
+                f"in a route or dependency. Common causes: the type is only "
+                f"imported inside an `if TYPE_CHECKING:` block (it must be "
+                f"available at runtime), or the class is defined after the "
+                f"route that references it."
+            )
         if annotation is type(None):
             return None
     return annotation

tests/test_dependencies_utils.py

@@ -1,3 +1,6 @@
+from typing import ForwardRef
+
+import pytest
 from fastapi.dependencies.utils import get_typed_annotation
 
 
@@ -6,3 +9,37 @@ def test_get_typed_annotation():
     annotation = "None"
     typed_annotation = get_typed_annotation(annotation, globals())
     assert typed_annotation is None
+
+
+def test_get_typed_annotation_unresolvable_string():
+    """A string annotation that can't be resolved should raise NameError."""
+    with pytest.raises(NameError, match="Could not resolve annotation 'NonExistent'"):
+        get_typed_annotation("NonExistent", {})
+
+
+def test_get_typed_annotation_unresolvable_forward_ref():
+    """A ForwardRef that can't be resolved should raise NameError."""
+    ref = ForwardRef("UnknownModel")
+    with pytest.raises(NameError, match="Could not resolve annotation 'UnknownModel'"):
+        get_typed_annotation(ref, {})
+
+
+def test_get_typed_annotation_resolvable_string():
+    """A string annotation that can be resolved should return the type."""
+    ns = {"int": int}
+    result = get_typed_annotation("int", ns)
+    assert result is int
+
+
+def test_get_typed_annotation_resolvable_forward_ref():
+    """A ForwardRef that can be resolved should return the type."""
+    ns = {"MyClass": str}
+    ref = ForwardRef("MyClass")
+    result = get_typed_annotation(ref, ns)
+    assert result is str
+
+
+def test_get_typed_annotation_non_string_passthrough():
+    """Non-string, non-ForwardRef annotations pass through unchanged."""
+    result = get_typed_annotation(int, {})
+    assert result is int

tests/test_unresolvable_annotations/apps/annotated_depends_late_type.py

@@ -0,0 +1,23 @@
+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=5)
+
+
+@app.get("/")
+async def read_root(potato: Annotated[Potato, Depends(get_potato)]):
+    return {"Hello": "World"}
+
+
+@dataclass
+class Potato:
+    color: str
+    size: int

tests/test_unresolvable_annotations/apps/type_checking_dep_return_type_safe.py

@@ -0,0 +1,19 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from fastapi import Depends, FastAPI
+
+if TYPE_CHECKING:
+    from sqlalchemy.orm import Session  # pragma: no cover
+
+app = FastAPI()
+
+
+def get_db() -> Session:
+    return "fake_db"  # type: ignore[return-value]
+
+
+@app.get("/")
+def read_root(db=Depends(get_db)) -> dict:
+    return {"db": str(db)}

tests/test_unresolvable_annotations/apps/type_checking_in_dep_param.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from fastapi import Depends, FastAPI
+
+if TYPE_CHECKING:
+    from pydantic import BaseModel  # pragma: no cover
+
+app = FastAPI()
+
+
+def get_thing(item: BaseModel) -> None: ...
+
+
+@app.get("/")
+def read_root(thing=Depends(get_thing)) -> dict: ...

tests/test_unresolvable_annotations/apps/type_checking_in_endpoint_param.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from fastapi import FastAPI
+
+if TYPE_CHECKING:
+    from pydantic import BaseModel  # pragma: no cover
+
+app = FastAPI()
+
+
+@app.get("/")
+def read_root(item: BaseModel) -> dict: ...

tests/test_unresolvable_annotations/apps/type_checking_in_return_type.py

@@ -0,0 +1,14 @@
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from fastapi import FastAPI
+
+if TYPE_CHECKING:
+    from pydantic import BaseModel  # pragma: no cover
+
+app = FastAPI()
+
+
+@app.get("/")
+def read_root() -> BaseModel: ...

tests/test_unresolvable_annotations/apps/type_declared_after_endpoint.py

@@ -0,0 +1,17 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+from fastapi import FastAPI
+
+app = FastAPI()
+
+
+@app.get("/")
+def read_root(potato: Potato) -> Potato: ...
+
+
+@dataclass
+class Potato:
+    color: str
+    size: int

tests/test_unresolvable_annotations/apps/valid_type_before_endpoint.py

@@ -0,0 +1,15 @@
+from __future__ import annotations
+
+from fastapi import FastAPI
+from pydantic import BaseModel
+
+app = FastAPI()
+
+
+class Item(BaseModel):
+    name: str
+
+
+@app.post("/")
+def create_item(item: Item) -> Item:
+    return item

tests/test_unresolvable_annotations/test_unresolvable_annotations.py

@@ -0,0 +1,74 @@
+"""Tests that unresolvable annotations raise clear errors at route registration time.
+
+Covers two common user mistakes:
+1. Using a type imported only under `if TYPE_CHECKING:` in endpoint/dependency params.
+2. Using a class that is defined after the route that references it.
+"""
+
+import importlib
+
+import pytest
+from fastapi.testclient import TestClient
+
+APP_PREFIX = "tests.test_unresolvable_annotations.apps"
+
+
+def test_type_declared_after_endpoint():
+    """A class used as an endpoint param before it's defined should fail."""
+    with pytest.raises(NameError, match="Could not resolve annotation 'Potato'"):
+        importlib.import_module(f"{APP_PREFIX}.type_declared_after_endpoint")
+
+
+def test_annotated_depends_late_type():
+    """Annotated[Potato, Depends(get_potato)] with Potato defined after the route should fail."""
+    with pytest.raises(NameError, match="Could not resolve annotation"):
+        importlib.import_module(f"{APP_PREFIX}.annotated_depends_late_type")
+
+
+def test_type_checking_import_in_endpoint_param():
+    """TYPE_CHECKING-only import used as an endpoint parameter type should fail."""
+    with pytest.raises(NameError, match="Could not resolve annotation 'BaseModel'"):
+        importlib.import_module(f"{APP_PREFIX}.type_checking_in_endpoint_param")
+
+
+def test_type_checking_import_in_dependency_param():
+    """TYPE_CHECKING-only import used as a dependency parameter type should fail."""
+    with pytest.raises(NameError, match="Could not resolve annotation 'BaseModel'"):
+        importlib.import_module(f"{APP_PREFIX}.type_checking_in_dep_param")
+
+
+def test_type_checking_import_in_return_type():
+    """TYPE_CHECKING-only import used as an endpoint return type should fail."""
+    with pytest.raises(NameError, match="Could not resolve annotation 'BaseModel'"):
+        importlib.import_module(f"{APP_PREFIX}.type_checking_in_return_type")
+
+
+def test_type_checking_dep_return_type_is_safe():
+    """TYPE_CHECKING-only import used ONLY as a dependency return type is safe.
+
+    FastAPI doesn't inspect dependency return types for schema generation,
+    so this pattern should not raise an error.
+    """
+    mod = importlib.import_module(f"{APP_PREFIX}.type_checking_dep_return_type_safe")
+    client = TestClient(mod.app)
+    response = client.get("/")
+    assert response.status_code == 200
+    assert response.json() == {"db": "fake_db"}
+
+
+def test_valid_type_before_endpoint():
+    """Sanity check: a type defined before the route works normally."""
+    mod = importlib.import_module(f"{APP_PREFIX}.valid_type_before_endpoint")
+    client = TestClient(mod.app)
+    response = client.post("/", json={"name": "test"})
+    assert response.status_code == 200
+    assert response.json() == {"name": "test"}
+
+
+def test_error_message_mentions_both_causes():
+    """The error message should mention both TYPE_CHECKING and late declaration."""
+    from fastapi.dependencies.utils import get_typed_annotation
+
+    with pytest.raises(NameError, match="TYPE_CHECKING") as exc_info:
+        get_typed_annotation("Unresolvable", {})
+    assert "defined after the route" in str(exc_info.value)