TypedDict inheritance doesn't work with get_type_hints and postponed evaluation of annotations across modules

[keithblaha]

BPO	41249
Nosy	@gvanrossum, @ambv, @Kronuz, @ilevkivskyi, @miss-islington, @septatrix, @keithblaha, @Fidget-Spinner
PRs	bpo-41249: Fix postponed annotations for TypedDict #27017 [3.10] bpo-41249: Fix postponed annotations for TypedDict (GH-27017) #27204 [3.9] bpo-41249: Fix postponed annotations for TypedDict (GH-27017) #27205

^{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 = <Date 2021-07-17.09:37:32.514>
created_at = <Date 2020-07-08.23:47:38.289>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.11', '3.7', 'library']
title = "TypedDict inheritance doesn't work with get_type_hints and postponed evaluation of annotations across modules"
updated_at = <Date 2021-07-17.09:37:32.513>
user = 'https://github.com/keithblaha'

bugs.python.org fields:

activity = <Date 2021-07-17.09:37:32.513>
actor = 'lukasz.langa'
assignee = 'none'
closed = True
closed_date = <Date 2021-07-17.09:37:32.514>
closer = 'lukasz.langa'
components = ['Library (Lib)']
creation = <Date 2020-07-08.23:47:38.289>
creator = 'keithblaha'
dependencies = []
files = []
hgrepos = []
issue_num = 41249
keywords = ['patch']
message_count = 18.0
messages = ['373360', '373752', '373758', '394729', '394738', '395279', '396296', '396919', '396923', '396926', '396946', '397690', '397692', '397694', '397695', '397710', '397711', '397712']
nosy_count = 8.0
nosy_names = ['gvanrossum', 'lukasz.langa', 'Kronuz', 'levkivskyi', 'miss-islington', 'Nils Kattenbeck', 'keithblaha', 'kj']
pr_nums = ['27017', '27204', '27205']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41249'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10', 'Python 3.11']

[keithblaha]

Copied from python/typing#737

I came across this issue while using inheritance to express required keys in a TypedDict, as is recommended by the docs.

It's most easily explained by a minimal example I cooked up. Let's say we have a module foo.py:

from __future__ import annotations

from typing import Optional
from typing_extensions import TypedDict

class Foo(TypedDict):
    a: Optional[int]

And another module bar.py:

from __future__ import annotations

from typing import get_type_hints
from foo import Foo

class Bar(Foo, total=False):
    b: int

print(get_type_hints(Bar))

Note that both foo.py and bar.py have adopted postponed evaluation of annotations (PEP-563) by using the __future__ import.

If we execute bar.py, we get the error message NameError: name 'Optional' is not defined.

This is due to the combination of:

get_type_hints relies on the MRO to resolve types: https://github.com/python/cpython/blob/3.7/Lib/typing.py#L970
TypedDict does not preserve the original bases, so Foo is not in the MRO for Bar:

typing/typing_extensions/src_py3/typing_extensions.py

Line 1652 in d79edde

     tp_dict = super(_TypedDictMeta, cls).__new__(cls, name, (dict,), ns) 

Thus, get_type_hints is unable to resolve the types for annotations that are only imported in foo.py.

I ran this example using typing_extensions 3.7.4.2 (released via #709) and Python 3.7.3, but it seems like this would be an issue using the current main branches of both repositories as well.

I'm wondering what the right approach is to tackling this issue. It is of course solvable by defining Bar in foo.py instead, but it isn't ideal or intuitive to always need to inherit from a TypedDict in the same module.

I was thinking that similarly to __required_keys__ and __optional_keys__, the TypedDict could preserve its original bases in a new dunder attribute, and get_type_hints could work off of that instead of MRO when it is dealing with a TypedDict. I would be willing to contribute the PRs to implement this if the design is acceptable, but am open to other ideas as well.

[ilevkivskyi]

I was thinking that similarly to __required_keys__ and __optional_keys__, the TypedDict could preserve its original bases in a new dunder attribute, and get_type_hints could work off of that instead of MRO when it is dealing with a TypedDict. I would be willing to contribute the PRs to implement this if the design is acceptable

TBH this is not very elegant, but I think you can go ahead with this (at least as a quick fix) since I don't see a better solution yet.

[keithblaha]

TBH this is not very elegant, but I think you can go ahead with this (at least as a quick fix) since I don't see a better solution yet.

Agreed, given that the current workaround of implementing them in the same module works I think I will stick with that while this is brainstormed further.

[Septatrix]

What is/was the initial reason to not preserve the MRO for a TypedDict?
The only thing which came to my mind would be instantiation performance but as annotations are not evaluated by default and on the right-hide side of assignment most people will use dict literals I am not sure if this is still relevant. Otherwise it might even be simpler to just preserve the original bases in TypedDict but please correct me if I overlooked something

[gvanrossum]

What is/was the initial reason to not preserve the MRO for a TypedDict?

Hm, I can't say for sure. I believe it had something to do with TypedDict instances being instances of dict at runtime, but I can't actually reconstruct the reason. Maybe it's written up in PEP-589, but I suspect not (I skimmed and couldn't find it). If you ask on typing-sig maybe David Foster (who contributed the initial idea and implementation) remembers.

[Septatrix]

I believe it had something to do with TypedDict instances being instances of dict at runtime, but I can't actually reconstruct the reason.

Hm that may be true.
My limited low-level Python knowledge leads me to believe that this could also be done using __new__ but I also read that most magic methods get called as type(Foo).__magic__(bar, ...) so that might not be possible.
(However also no methods can be declared on a TypedDict class so that might not be a problem?)

Maybe it's written up in PEP-589, but I suspect not (I skimmed and couldn't find it).

I read it completely and could not find anything

If you ask on typing-sig maybe David Foster (who contributed the initial idea and implementation) remembers.

I asked here on typing-sig but did not yet get any responses.

[gvanrossum]

Note that this issue is now only a problem of you use from __future__ import annotations -- we rolled the default behavior for 3.10 back to what it was in 3.9.

I am out of time to argue about why we chose this behavior, alas.

[Kronuz]

The way I fixed this is I added __forward_module__ to typing.ForwardRef, so that it can resolve the forward reference with the same globals as the ones specified by the module in __forward_module__. TypedDict's metaclass should then pass the dictionary’s module name to the annotations’ forward references via the added module’s keyword argument in typing._type_check(). I can work in a pull request with this solution and discuss any potential problems.

[Septatrix]

The way I fixed this is I added __forward_module__ to typing.ForwardRef, so that it can resolve the forward reference with the same globals as the ones specified by the module in __forward_module__. TypedDict's metaclass should then pass the dictionary’s module name to the annotations’ forward references via the added module’s keyword argument in typing._type_check(). I can work in a pull request with this solution and discuss any potential problems.

While this seems like a good solution I would still like to figure out why TypedDict do not preserve MRO. Because for now I have not found a reason nor did someone on the typing-sig mailinglist have a clue. Should there (no longer) be a reason for this then this problem has a trivial solution (just re-add the MRO and use that).