Another solution if you write fully-typed Python is to use typing.Literal:
calc(x, y, mode="gain")
where the function is defined as
def calc(
x: float,
y: float,
*,
mode: Literal["gain", "render"],
) -> float:
...
This is IMO one of the best things type annotations provide. Combined with Protocol you can write much better structured code with little or no runtime costs.
