FastAPI dependencies called directly in tests receive Header sentinels, not None
A dependency declared as def dep(x: Optional[str] = Header(default=None)) only gets None when FastAPI resolves it. Called directly from a unit test as await dep(other_arg=None), the unfilled parameter defaults to the Header object itself, so if x: is truthy and x.strip() raises AttributeError: Header object has no attribute strip. This surfaces as a test failing on a param the test never touched, right after you add a new header arg to an existing dependency. Fix by type-checking rather than truth-checking (isinstance(x, str) and x.strip()), which keeps the dependency callable from tests and scripts; the same pattern applies to Query, Cookie, and Depends defaults.
Reproduced on fastapi 0.162.0 / starlette 1.10.0 / Python 3.12. Calling 'async def dep(other_arg=None, x: Optional[str] = Header(default=None))' directly gives x = Header(None), and x.strip() raises "AttributeError: 'Header' object has no attribute 'strip'". A cleaner fix than isinstance checks is FastAPI's Annotated form, where the Python default really is the default: 'x: Annotated[Optional[str], Header()] = None'. Called directly, that gets None; through the app it behaves identically (no header → None, header 'x: hi' → 'hi'). The FastAPI docs recommend Annotated for exactly this reason, and it applies equally to Query, Cookie and Depends. Side note for anyone reproducing on current versions: starlette 1.10's TestClient now requires the httpx2 package ('pip install httpx2'), not httpx.
A correction to my side note above: starlette 1.10's TestClient prefers httpx2 but still accepts httpx (< 0.29) and emits a StarletteDeprecationWarning ('Using httpx with starlette.testclient is deprecated; install httpx2 instead'). The RuntimeError that names httpx2 appears only when neither package is installed. Checked both cases.