Specify length of Sequence or List with Python typing module

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.

*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.

Solution 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.

Solution 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.

Solution 3:

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

Annotated[List[float], 3]

Solution 4:

When also confronted with the same problem, I was not happy seeing Martijn Pieters answer. Since I wanted a "fast" and "easy" way to solve this problem.

So I tried the other suggestions listed here first.

Note: I used VSCode with Pylance as Language Server

Zaffys answer was my favorite

def demystify(mystery: Annotated[Tuple[int], 6]):
    a, b, c, d, e, f = mystery
    print(a, b, c, d, e, f)

Hint for the function then looks like this: demystify: (mystery: Tuple[int]) -> None Also I get a Pylance Error Tuple size mismatch: expected 6 but received for the line a, b, c, d, e, f = mystery

Next I tried Tuple[6 * (int, )] which was mentioned by balu in the comments of Martijn Pieters answer

def demystify(mystery: Tuple[6 * (int,)]):
    a, b, c, e, f, g = mystery
    print(a, b, c, e, f, g)

Resulting in the same Pylance Error as before. Hint for the function was this: demystify: (mystery: Tuple[Tuple[Type[int], ...]]) -> None

Going back to writing down the expected length:

def demystify(mystery: Tuple[int, int, int, int, int, int]):
    a, b, c, e, f, g = mystery
    print(a, b, c, e, f, g)

This resolved the Pylance Error, and got me a "clear" function hint: demystify: (mystery: Tuple[int, int, int, int, int, int]) -> None

But just like John Brodie, I was not happy with this solution.

Now back to the, at first, unwanted answer:

class MysteryType(NamedTuple):
    a: int
    b: int
    c: int
    d: int
    e: int
    f: int
    g: int

def demystify(mystery: MysteryType):
    print(*mystery)

The function hint now seems more mystic: demystify: (mystery: MysteryType) -> None but creating a new MysteryType gives me all the information I need: (a: int, b: int, c: int, d: int, e: int, f: int, g: int)

Also I can use the MysteryType in other methods and functions without the need of counting the type hints.

So, to make a long story short and paraphrase the Zen of Python:

NamedTuples are one honking great idea -- let's do more of those!