Specify length of Sequence or List with Python typing module

You can't. A list is a mutable, variable length structure. If you need a fixed-length structure, use a tuple instead:

Tuple[float, float, float, float, float, float, float, float, float, float]

Or better still, use a named tuple, which has both indices and named attributes:

class BunchOfFloats(NamedTuple):    foo: float    bar: float    baz: float    spam: float    ham: float    eggs: float    monty: float    python: float    idle: float    cleese: float

A list is simply the wrong data type for a fixed-length data structure.

So far, only tuples support specifying a fixed number of fields and it has no short-cut for a fixed number of repetitions.

Here's the definition and docstring from the typing module:

class Tuple(tuple, extra=tuple, metaclass=TupleMeta):    """Tuple type; Tuple[X, Y] is the cross-product type of X and Y.    Example: Tuple[T1, T2] is a tuple of two elements corresponding    to type variables T1 and T2.  Tuple[int, float, str] is a tuple    of an int, a float and a string.    To specify a variable-length tuple of homogeneous type, use Tuple[T, ...].    """    __slots__ = ()    def __new__(cls, *args, **kwds):        if _geqv(cls, Tuple):            raise TypeError("Type Tuple cannot be instantiated; "                            "use tuple() instead")        return _generic_new(tuple, cls, *args, **kwds)

Since lists are a mutable, variable-length type, it doesn't make any sense to use a type declaration to specify a fixed size.

Annotated can be handy here. It allows you to specify arbitrary metadata to type hints:

Annotated[List[float], 3]