Unioned types should be hashable: not documented?

[faph]

Documentation

It seems that typing.Union[X, Y] as well as X | Y type hints require X and Y to be hashable.

This is not documented as far as I can see.

Considering this example:

@dataclass
class ValueRange:
    lo: int
    hi: int

T1 = Annotated[int, ValueRange(-10, 5)]

This fails when unioned:

>>> Annotated[int, ValueRange(-10, 5)] | None
(...)
  File "/usr/lib64/python3.11/typing.py", line 1375, in __or__
    return Union[self, right]
           ~~~~~^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/typing.py", line 358, in inner
    return func(*args, **kwds)
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/typing.py", line 481, in __getitem__
    return self._getitem(self, parameters)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/typing.py", line 695, in Union
    parameters = _remove_dups_flatten(parameters)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/typing.py", line 326, in _remove_dups_flatten
    return tuple(_deduplicate(params))
                 ^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib64/python3.11/typing.py", line 301, in _deduplicate
    all_params = set(params)
                 ^^^^^^^^^^^
  File "/usr/lib64/python3.11/typing.py", line 2151, in __hash__
    return hash((self.__origin__, self.__metadata__))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unhashable type: 'ValueRange'

Should this be documented say here: https://docs.python.org/3/library/typing.html?highlight=typing#typing.Annotated ?

And examples here should use frozen dataclasses: https://docs.python.org/3/library/typing.html?highlight=typing#typing.Annotated ?

Linked PRs

• gh-112281: Allow Union with unhashable Annotated metadata #112283
• [3.12] gh-112281: Allow Union with unhashable Annotated metadata (GH-112283) #116213
• [3.11] gh-112281: Allow Union with unhashable Annotated metadata (GH-112283) #116288

[sobolevn]

PEPs https://peps.python.org/pep-0593/#specification and https://peps.python.org/pep-0604/ do not say anything about requirement of items to be hashable. I think that this is a bug: unhashable items must be allowed.

I think that we need to rework our _deduplicate function. I have an idea on how to do it :)

Thanks for the report! 👍

[sobolevn]

This is more complex than I initially thought, because _UnionGenericAlias also uses set to compare results. So, we need a slow way of doing it as well. See

cpython/Lib/typing.py

Line 1536 in ce1096f

return set(self.__args__) == set(other.__args__)