Create a typing.Annotated instance with variadic args in older Python

The question is simply how to reproduce this:

import typing

a = [1, 2, 3]
cls = typing.Annotated[int, *a]

or even this:

import typing

cls1 = typing.Annotated[int, 1, 2, 3]
unann_cls = typing.get_args(cls1)[0]
metadata = cls1.__metadata__
cls2 = typing.Annotated[unann_cls, *metadata]

in Python 3.9-3.10. Annotated[int, *a] is a syntax error in <3.11 so typing_extensions.Annotated shouldn’t help here. "Nested Annotated types are flattened" suggests the following code:

cls2 = unann_cls
for m in metadata:
    cls2 = typing.Annotated[cls2, m]

and it seems to work but surely there should be a more clean way?

>Solution :

In the expression

obj[1, 2, 3]

1, 2, 3 is just a tuple literal. It’s exactly the same as

x = 1, 2, 3
obj[x]

We’re simply passing the tuple (1, 2, 3) to __getitem__.

To get the behavior you want, you can wrap the tuple literal with parentheses so that the syntax for sequence unpacking inside a tuple literal kicks in:

>>> typing.Annotated[(int, *a)]
typing.Annotated[int, 1, 2, 3]