With Union, forward references in strings get converted to ForwardRef. To wit:

>>> from typing import Union

>>> Alias = Union[str, "Foo"]

>>> Alias

typing.Union[str, ForwardRef('Foo')]

With the new UnionType, the following is not valid:

>>> Alias = str | "Foo"

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: unsupported operand type(s) for |: 'type' and 'str'

The following is also invalid:

>>> from typing import ForwardRef

>>> Alias = str | ForwardRef("Foo")

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

TypeError: unsupported operand type(s) for |: 'type' and 'ForwardRef'

Having the whole type alias as a string doesn't quite make sense to me, but maybe it's in fact correct:

>>> Alias = "str | Foo"

>>> Alias

'str | Foo'

I see this open BPO, which was discussed in 2021:

Any suggestions on how else to make forward references in type aliases with the new UnionType?