In our FastAPI tests we switch between different test users that we have as fixtures a lot, and I found that sub-classing TestClient is a nice way to switch between them via client.as_user(user_42).get(...) if I do this:

class MyTestClient(TestClient):
    """Enhanced test client that can authenticate as different users via bearer tokens."""

    def as_user(self, user: User) -> Self:
        self.headers.update(create_access_token(user).headers)
        return self

@pytest.fixture
def client(db, test_user) -> Generator[MyTestClient]:
    app.dependency_overrides[get_session] = lambda: db
    with MyTestClient(app) as test_client:
        yield test_client.as_user(test_user)

    app.dependency_overrides.clear()

Unfortunately pyright gives this error incorrectly claiming that client.as_user doesn't exist:

 error: Cannot access attribute "as_user" for class "TestClient"
    Attribute "as_user" is unknown (reportAttributeAccessIssue)

The problem is that you have a hard-coded return type annotation on TestClient.__enter__ that doesn't work with sub-classes:

I gather you can't use typing.Self yet?
Is there some other way to fix that type annotation?

For now I'll do a manual typing.cast as workaround.