GenericAlias does not support nested type variables

[serhiy-storchaka]

BPO	40408
Nosy	@gvanrossum, @serhiy-storchaka, @ilevkivskyi
PRs	bpo-40408: Fix support of nested type variables in GenericAlias. #19836

^{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 = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2020-05-04.08:00:19.205>
created_at = <Date 2020-04-27.15:57:57.150>
labels = ['interpreter-core', 'type-bug', '3.9']
title = 'GenericAlias does not support nested type variables'
updated_at = <Date 2020-05-04.08:30:23.949>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2020-05-04.08:30:23.949>
actor = 'levkivskyi'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2020-05-04.08:00:19.205>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2020-04-27.15:57:57.150>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 40408
keywords = ['patch']
message_count = 6.0
messages = ['367433', '367454', '367506', '367944', '368014', '368019']
nosy_count = 3.0
nosy_names = ['gvanrossum', 'serhiy.storchaka', 'levkivskyi']
pr_nums = ['19836']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40408'
versions = ['Python 3.9']

[serhiy-storchaka]

While trying to replace typing._GenericAlias with GenericAlias I have found that the latter does not support nested type variables.

>>> from typing import *
>>> T = TypeVar('T')
>>> X = List[List[T]]
>>> X.__parameters__
(~T,)
>>> X[int]
typing.List[typing.List[int]]
>>> Y = list[list[T]]
>>> Y.__parameters__
()
>>> Y[int]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: There are no type variables left in list[list[~T]]

[gvanrossum]

Good catch. Is there a reasonable fix?

[serhiy-storchaka]

Not yet. If you don't prefer to fix it yourself, I'll do it as soon as I have time.

[serhiy-storchaka]

There is a difference between PR 19836 and the typing module in handling nested unsubscribed generic aliases:

>>> from typing import *
>>> T = TypeVar('T')
>>> D1 = Dict[T, List]
>>> D2 = dict[T, List]
>>> D1.__parameters__
(~T,)
>>> D1[int]
typing.Dict[int, typing.List[~T]]
>>> D1[int].__parameters__
(~T,)
>>> D1[int][str]
typing.Dict[int, typing.List[str]]
>>> D1[int, str]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/serhiy/py/cpython/Lib/typing.py", line 267, in inner
    return func(*args, **kwds)
  File "/home/serhiy/py/cpython/Lib/typing.py", line 686, in __getitem__
    _check_generic(self, params)
  File "/home/serhiy/py/cpython/Lib/typing.py", line 221, in _check_generic
    raise TypeError(f"Too {'many' if alen > elen else 'few'} parameters for {cls};"
TypeError: Too many parameters for typing.Dict[~T, typing.List]; actual 2, expected 1
>>> D2.__parameters__
(~T, ~T)
>>> D2[int]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Too few arguments for dict[~T, typing.List]
>>> D2[int, str]
dict[int, typing.List[str]]

But this behavior is not specified and is not covered by tests.

[serhiy-storchaka]

New changeset 41a6458 by Serhiy Storchaka in branch 'master':
bpo-40408: Fix support of nested type variables in GenericAlias. (GH-19836)
41a6458

[ilevkivskyi]

But this behavior is not specified and is not covered by tests.

FWIW, to be most close to the static type checkers behavior, both D[int][str] and D[int, str] should fail for D = Dict[T, List]. Not important however, since this is a really rare corner case I think.