Improve error message for missing dependency type annotation

fastapi/dependencies/utils.py

@@ -46,7 +46,7 @@ from fastapi.concurrency import (
     contextmanager_in_threadpool,
 )
 from fastapi.dependencies.models import Dependant
-from fastapi.exceptions import DependencyScopeError
+from fastapi.exceptions import DependencyScopeError, FastAPIError
 from fastapi.logger import logger
 from fastapi.security.oauth2 import SecurityScopes
 from fastapi.types import DependencyCacheKey
@@ -109,7 +109,9 @@ def ensure_multipart_is_installed() -> None:
             raise RuntimeError(multipart_not_installed_error) from None
 
 
-def get_parameterless_sub_dependant(*, depends: params.Depends, path: str) -> Dependant:
+def get_parameterless_sub_dependant(
+    *, depends: params.Depends, path: str
+) -> Dependant:
     assert callable(depends.dependency), (
         "A parameter-less dependency must have a callable dependency"
     )
@@ -121,6 +123,7 @@ def get_parameterless_sub_dependant(*, depends: params.Depends, path: str) -> De
         call=depends.dependency,
         scope=depends.scope,
         own_oauth_scopes=own_oauth_scopes,
+        enforce_annotation=False,
     )
 
 
@@ -248,6 +251,13 @@ def get_typed_return_annotation(call: Callable[..., Any]) -> Any:
     return get_typed_annotation(annotation, globalns)
 
 
+def _dependency_defines_type(depends: params.Depends) -> bool:
+    dependency = depends.dependency
+    if inspect.isclass(dependency):
+        return True
+    return get_typed_return_annotation(dependency) is not None
+
+
 def get_dependant(
     *,
     path: str,
@@ -257,6 +267,7 @@ def get_dependant(
     parent_oauth_scopes: Optional[list[str]] = None,
     use_cache: bool = True,
     scope: Union[Literal["function", "request"], None] = None,
+    enforce_annotation: bool = True,
 ) -> Dependant:
     dependant = Dependant(
         call=call,
@@ -281,6 +292,15 @@ def get_dependant(
         )
         if param_details.depends is not None:
             assert param_details.depends.dependency
+            if (
+                enforce_annotation
+                and param.annotation is inspect.Signature.empty
+                and not _dependency_defines_type(param_details.depends)
+            ):
+                raise FastAPIError(
+                    f'Dependency parameter "{param_name}" must have a type annotation. '
+                    f'For example: `{param_name}: SomeType = Depends(...)`'
+                )
             if (
                 (dependant.is_gen_callable or dependant.is_async_gen_callable)
                 and dependant.computed_scope == "request"
@@ -303,6 +323,7 @@ def get_dependant(
                 parent_oauth_scopes=current_scopes,
                 use_cache=param_details.depends.use_cache,
                 scope=param_details.depends.scope,
+                enforce_annotation=False,
             )
             dependant.dependencies.append(sub_dependant)
             continue
@@ -315,7 +336,11 @@ def get_dependant(
                 f"Cannot specify multiple FastAPI annotations for {param_name!r}"
             )
             continue
-        assert param_details.field is not None
+        if enforce_annotation and param_details.field is None:
+            raise FastAPIError(
+                f'Dependency parameter "{param_name}" must have a type annotation. '
+                f'For example: `{param_name}: SomeType = Depends(...)`'
+            )
         if isinstance(param_details.field.field_info, params.Body):
             dependant.body_params.append(param_details.field)
         else:
@@ -609,6 +634,7 @@ async def solve_dependencies(
                 name=sub_dependant.name,
                 parent_oauth_scopes=sub_dependant.oauth_scopes,
                 scope=sub_dependant.scope,
+                enforce_annotation=False,
             )
 
         solved_result = await solve_dependencies(

fastapi/routing.py

@@ -456,7 +456,10 @@ class APIWebSocketRoute(routing.WebSocketRoute):
         self.dependencies = list(dependencies or [])
         self.path_regex, self.path_format, self.param_convertors = compile_path(path)
         self.dependant = get_dependant(
-            path=self.path_format, call=self.endpoint, scope="function"
+            path=self.path_format,
+            call=self.endpoint,
+            scope="function",
+            enforce_annotation=False,
         )
         for depends in self.dependencies[::-1]:
             self.dependant.dependencies.insert(

tests/test_dependency_overrides.py

@@ -2,6 +2,7 @@ from typing import Optional
 
 import pytest
 from fastapi import APIRouter, Depends, FastAPI
+from fastapi.exceptions import FastAPIError
 from fastapi.testclient import TestClient
 
 app = FastAPI()
@@ -389,3 +390,21 @@ def test_override_with_sub_router_decorator_depends_k_bar():
     assert response.status_code == 200
     assert response.json() == {"in": "router-decorator-depends"}
     app.dependency_overrides = {}
+
+
+def test_missing_type_annotation_dependency():
+    local_app = FastAPI()
+
+    def get_db():
+        return "db"
+
+    with pytest.raises(FastAPIError) as exc:
+        @local_app.get("/bad")
+        def bad(dep=Depends(get_db)):
+            return dep
+        TestClient(local_app)
+
+    msg = str(exc.value)
+    assert "Dependency parameter" in msg
+    assert "type annotation" in msg
+    assert "Depends" in msg