Type hints when unpacking a tuple?

Question:

Is it possible to use type hinting when unpacking a tuple? I want to do this, but it results in a SyntaxError:

from typing import Tuple

t: Tuple[int, int] = (1, 2)
a: int, b: int = t
#     ^ SyntaxError: invalid syntax
Asked By: Cai

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Source

Answers:

According to PEP-0526, you should annotate the types first, then do the unpacking

a: int
b: int
a, b = t
Answered By: Patrick Haugh

In my case i use the typing.cast function to type hint an unpack operation.

t: tuple[int, int] = (1, 2)
a, b = t

# type hint of a -> Literal[1]
# type hint of b -> Literal[2]

By using the cast(new_type, old_type) you can cast those ugly literals into integers.

from typing import cast

a, b = cast(tuple[int, int], t)

# type hint of a -> int
# type hint of b -> int

This can be useful while working with Numpy NDArrays with Unknown types

# type hint of arr -> ndarray[Unknown, Unknown]

a, b = cast(tuple[float, float], arr[i, j, :2]

# type hint of a -> float
# type hint of b -> float
Answered By: Jorge Ruiz