Subclassing of annotated types does not always work

[serhiy-storchaka]

BPO	44969
Nosy	@gvanrossum, @serhiy-storchaka, @Fidget-Spinner

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-08-21.07:41:02.034>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'Subclassing of annotated types does not always work'
updated_at = <Date 2021-08-21.07:47:25.292>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2021-08-21.07:47:25.292>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-08-21.07:41:02.034>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 44969
keywords = []
message_count = 2.0
messages = ['400021', '400022']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'kj']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44969'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

Linked PRs

• gh-89132: Fix subclassing of annotated types #155360

[serhiy-storchaka]

It works only with simple types

>>> class X(Annotated[list, 'annotation']): pass
... 

But not with type aliases

>>> class X(Annotated[List[int], 'annotation']): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: _GenericAlias.__init__() takes 3 positional arguments but 4 were given
>>> class X(Annotated[list[int], 'annotation']): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: GenericAlias expected 2 arguments, got 3

And even if the original type is not subclassable, the error message is not always clear:

>>> class X(Annotated[Union[int, str], 'annotation']): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: _GenericAlias.__init__() takes 3 positional arguments but 4 were given
>>> class X(Annotated[int | str, 'annotation']): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: _GenericAlias.__init__() takes 3 positional arguments but 4 were given

[serhiy-storchaka]

I think that __mro_entries__() for annotated types should return the result of __mro_entries__() for the original type if it has __mro_entries__. But details can be complicated, especially if user-defined generic types are involved.

Alternatively, __build_class__ can call __mro_entries__() repeatedly for the result of __mro_entries__(). I do not know what is better.