No error for singledispatch on generic types

[wrobell]

Please consider

from functools import singledispatch
import typing as tp

@singledispatch
def f(x: tp.Any) -> None: pass

@f.register
def _f_list(x: list[int]) -> None: pass

if __name__ == '__main__':
    f([1, 2, 3])

Then

$ mypy sp.py
Success: no issues found in 1 source file

# on import no error as well, so Mypy raising an error would be very useful
$ python -c 'import sp'

On runtime

$ python sp.py
Traceback (most recent call last):
  File "/usr/lib/python3.10/functools.py", line 831, in dispatch
    impl = dispatch_cache[cls]
  File "/usr/lib/python3.10/weakref.py", line 416, in __getitem__
    return self.data[ref(key)]
KeyError: <weakref at 0x7f390d605c60; to 'type' at 0x7f390e07bd00 (list)>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.10/functools.py", line 834, in dispatch
    impl = registry[cls]
KeyError: <class 'list'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/wrobell/projects/rbfly/sp.py", line 11, in <module>
    f([1, 2, 3])
  File "/usr/lib/python3.10/functools.py", line 878, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
  File "/usr/lib/python3.10/functools.py", line 836, in dispatch
    impl = _find_impl(cls, registry)
  File "/usr/lib/python3.10/functools.py", line 783, in _find_impl
    mro = _compose_mro(cls, registry.keys())
  File "/usr/lib/python3.10/functools.py", line 744, in _compose_mro
    types = [n for n in types if is_related(n)]
  File "/usr/lib/python3.10/functools.py", line 744, in <listcomp>
    types = [n for n in types if is_related(n)]
  File "/usr/lib/python3.10/functools.py", line 743, in is_related
    and issubclass(cls, typ))
TypeError: issubclass() argument 2 cannot be a parameterized generic

Some context (in general we need to be better at distinguishing between Python types and annotation types)

• https://bugs.python.org/issue34498
• TypeForm[T]: Spelling for regular types (int, str) & special forms (Union[int, str], Literal['foo'], etc) #9773

[AlexWaygood]

^Note that the runtime error message has been recently improved by python/cpython#30050, which will appear in the next micro versions of 3.9, 3.10 and 3.11.

The topic of whether to ever support generic aliases in singledispatch was also recently discussed, and rejected, in a third BPO issue here: https://bugs.python.org/issue46191

[AlexWaygood]

Also, note that three pseudo-types will be supported by singledispatch in 3.11: types.UnionType, typing.Union, and typing.Optional: https://bugs.python.org/issue46014

[hauntsaninja]

If we wanted to add more special casing for singledispatch, could also add some disallow-any-generic special casing (see https://bugs.python.org/issue46191 )

Edit: Oh I see Alex already linked that issue. But yeah, another potential mypy "feature"