Specify length of Sequence or List with Python typing module

Question

I'm giving the Python typing module a shot.

I know that it's valid to specify the length of a List like the following*:

List[float, float, float]   # List of 3 floats <-- NOTE: this is not valid Python 

Is there any shorthand for longer lists? What if I want to set it to 10 floats?

List[float * 10]   # This doesn't work. 

Any idea if this is possible, this would be handy.

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*NOTE: It turns out that supplying multiple arguments to Sequence[] (and its subclasses) in this manner is currently NOT valid Python. Furthermore, it is currently not possible to specify a Sequence length using the typing module in this way.

Answer 1

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.

Answer 2

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.