Open craigh92 opened 3 years ago
Experimenting with this idea, I found a possible solution using the existing API.
Create type-hint for a fixed-length homogeneous tuple directly with types.GenericAlias
.
from types import GenericAlias
from typing import TypeVar
T = TypeVar('T')
EightTuple = GenericAlias(tuple, (T,)*8)
eight_ints: EightTuple[int] = (1, 2, 3, 4, 5, 6, 7, 8)
eight_strs: EightTuple[str] = ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h')
At first, I tried manipulating tuple
s directly:
>>> (int,)*10
(<class 'int'>, <class 'int'>, <class 'int'>, <class 'int'>, <class 'int'>, <class 'int'>, <class 'int'>, <class 'int'>, <class 'int'>, <class 'int'>)
>>> from typing import TypeVar
>>> T = TypeVar('T')
>>> TenTuple = (T,)*10
>>> TenTuple
(~T, ~T, ~T, ~T, ~T, ~T, ~T, ~T, ~T, ~T)
However, I got stuck at this point because:
>>> TenTuple[int]
Traceback (most recent call last):
File "<input>", line 1, in <module>
TypeError: tuple indices must be integers or slices, not type
I tried other variations on this theme with typing.Generic
, and kept getting the same error.
I also made a desperate attempt at a tuple metatype, but that failed miserably.
Then I looked at
>>> type(tuple[int])
<class 'types.GenericAlias'>
Which got me thinking, maybe we can just create our own alias type directly:
>>> from types import GenericAlias
>>> EightTuple = GenericAlias(tuple, (T,)*8)
>>> EightTuple
tuple[~T, ~T, ~T, ~T, ~T, ~T, ~T, ~T]
>>> EightTuple[int]
tuple[int, int, int, int, int, int, int, int]
In Julia, this type alias is called NTuple
:
julia> NTuple{3,Int}
Tuple{Int64, Int64, Int64}
Experimenting with this idea, I found a possible solution using the existing API.
Summary
Create type-hint for a fixed-length homogeneous tuple directly with
types.GenericAlias
.from types import GenericAlias from typing import TypeVar T = TypeVar('T') EightTuple = GenericAlias(tuple, (T,)*8) eight_ints: EightTuple[int] = (1, 2, 3, 4, 5, 6, 7, 8) eight_strs: EightTuple[str] = ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h')
Did something change? I am getting:
error: Variable "test.EightTuple" is not valid as a type [valid-type]
(mypy 1.4.1)
Experimenting with this idea, I found a possible solution using the existing API.
Summary
Create type-hint for a fixed-length homogeneous tuple directly with
types.GenericAlias
.from types import GenericAlias from typing import TypeVar T = TypeVar('T') EightTuple = GenericAlias(tuple, (T,)*8) eight_ints: EightTuple[int] = (1, 2, 3, 4, 5, 6, 7, 8) eight_strs: EightTuple[str] = ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h')
Did something change? I am getting:
error: Variable "test.EightTuple" is not valid as a type [valid-type]
(mypy 1.4.1)
This was two years a go, so I don't remember exactly how deep I got with this. I seem to recall only playing around with GenericAlias
, but not validating the resulting type in mypy. It would be nice for these to validate though. I suspect the issue is that the code has to execute, and mypy only looks at static code, but I'm not 100% sure on that.
A thought on syntax: I just stumbled on the similar foo: tuple[int, int, int]
problem, and my naive/intuitive guess was foo: tuple[int]*3
or foo: tuple[int*3]
, based on the familiar (7,)*3
syntax.
Given that T1 | T2 | ... | Tn
is now valid syntax for union types, I think the natural extension for product types (i.e., tuples) would be T1 & T2 & ... & Tn
. For very long homogeneous tuples, I'd say either (T,) * n
or T ** n
would make sense.
Experimenting with this idea, I found a possible solution using the existing API.
Summary
Create type-hint for a fixed-length homogeneous tuple directly with
types.GenericAlias
.from types import GenericAlias from typing import TypeVar T = TypeVar('T') EightTuple = GenericAlias(tuple, (T,)*8) eight_ints: EightTuple[int] = (1, 2, 3, 4, 5, 6, 7, 8) eight_strs: EightTuple[str] = ('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h')
This doesn't work for me. This code does not issue a typing warning:
from types import GenericAlias
from typing import TypeVar
T = TypeVar('T')
Pair = GenericAlias(tuple, (T,)*2)
def foo(a: Pair[int]) -> None:
print(a[0])
foo((1, 3, 3))
# Expected a typing warning ("Expected type 'tuple[int, int]', got 'tuple[int, int, int]' instead"), but did not get one.
# Changing the type hint of `a` in `foo` to 'tuple[int, int]' does issue this warning, as expected.
I'm using Python 3.10, mypy==1.11.2, PyCharm community 2024.2.
Feature
A concise way to hint that a sequence of homogenous types is a set length. i.e The items in the sequence are all of the same type, the sequence is iterable, and cannot grow larger or smaller.
Pitch
Currently, the recommended way to add type hints to fixed-length sequences is to use Tuples 1. i.e
For longer lists, this repetition is tedious and prone to error. It would be nice to have a shorthand for defining homogeneous tuples. e.g
Although the name
Tuple
does not signal intent very well. The real thing that we are trying to signal is that this is a sequence of floats of length 10. It does not necessarily have to be a Tuple, it just has to be iterable, and have 10 items, that cannot grow larger or smaller. If I wanted to pass the function a List, this should also pass static type checkers, as long as it has 10 floats.Possible Implementation
I am not a core python developer, I have never even looked at the implementation of the typing module, and I'm not even that good of a python developer, so please forgive me if this is nonsense.
In Python 3.10 or newer [2] we will be able to use the unpack operator in subscript. So we might be able to do something like:
Using TypeVars, we could generalize this function to accept 10 of any type (as long as they are all the same type)
Following this logic, perhaps a generic type hint for Fixed Length Homogeneous Sequences could be implemented with
FixedLengthHomogeneousSequence
is a bit wordy. So perhapsFixedList
would be a better name.[2] (I think this is true). Pylance gave me an error saying this was the case. I think it's related to https://www.python.org/dev/peps/pep-0622/