Not positive this is a bug, but it seems like ForwardRef should support the pipe (|) operator for indicating Unions in Python 3.10:

Python 3.10.0 (default, Oct  4 2021, 22:09:55) [GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from typing import ForwardRef
>>> ForwardRef('asdf') | ForwardRef('fdsa')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for |: 'ForwardRef' and 'ForwardRef'
>>> int | str
int | str
>>>

--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -719,6 +719,12 @@ def __eq__(self, other):
def __hash__(self):
return hash(self.__forward_arg__)

+ def __or__(self, other):
+ return Union[self, other]
+
+ def __ror__(self, other):
+ return Union[other, self]
+
def __repr__(self):
return f'ForwardRef({self.__forward_arg__!r})'

This can be easily fixed, but I am waiting for guido and ken to check whether this is an intentioned operation.

I suppose this is useful for programs that use runtime annotations (e.g. pydantic?). I don’t mind adding this.

ForwardRef is for internal use, and writing ForwardRef('fdsa') is discouraged, as the docs say:

This class should not be instantiated by a user.

However, I don't see any cons from adding union support, so let's do it!

Note that a workaround exists for OP's example. But it may not be sufficient for complex runtime libraries:

ForwardRef('asdf | fdsa')

New changeset 15ad52f by Dong-hee Na in branch 'main':
bpo-45489: Update ForwardRef to support | operator. (GH-28991)
15ad52f

@guido

Do we need to create a backport for 3.10?
Ken and I are waiting for your comment :)
#28991 (comment)

No. :-)

Thanks @corona10 for the patch.

@bobbeyreese
The new behavior is available in 3.11 and up. 3.10.x won't be supporting the | operator.