Reason given for disallowing non-concrete subtype assignment is unsound
Nobody has claimed this yet.
Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 25/100
- Issue type
- Documentation
- Clarity
- Mostly clear
- Activity status
- Stale
- Tech stack
- python
- Domain
- documentation
Research direction
Start with the linked type-and-class-objects-vs-protocols section in docs/spec/protocol.rst, then read the linked typing discussion and mypy issue for the surrounding rationale. The issue presents competing directions rather than a settled change; done would require an agreed specification decision followed by updates to the reasoning and associated examples.
Written by the indexing model from the issue text.
Description
Issue
According to the discussion https://github.com/python/typing/discussions/1305, “type checkers allow incompatible __init__ overrides, because flagging them would be too disruptive.”
Accepting above as de facto, the example given as the main reason for disallowing non-concrete subtype assignment in the following section of the spec is unsound:
https://github.com/python/typing/blob/e08290b70f58df509f998cbbe09a8e65abb57a9b/docs/spec/protocol.rst#type-and-class-objects-vs-protocols
class Proto(Protocol):
@abstractmethod
def meth(self) -> int:
...
class Concrete:
def meth(self) -> int:
return 42
def fun(cls: type[Proto]) -> int:
return cls().meth() # ???
fun(Proto) # Why should this error?
fun(Concrete) # OK
var: Type[Proto]
var = Proto # Why should this error?
var = Concrete # OK
var().meth() # ???
(credit https://github.com/python/mypy/issues/4717#issuecomment-1978239641 for pointing out the contradiction)
Thoughts
One radical approach would be to remove the concreteness rule from the spec altogether. The type of Proto is type[Proto], and if the constructor compatibility is not checked, there is little reason to disallow it from being assigned to variables annotated as type[Proto].
If that is too radical, the spec can stay as is but the reasoning and associated examples should be clearly marked ‘historical’ and no longer valid. That way, we can avoid any immediate changes in practice, but at the same time encourage discussions towards appropriate future specs.
- Dominant language
- Python
- Stars
- 1.8k
- Forks
- 302
- Avg merge
- 23h
- Merged PRs (30d)
- 8
Contributor guide
No contributing guide indexed for this repository
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
More from python/typing
-
topic: typing spec
Difficulty 2/5 1-3 hours Newbie friendliness 72/100
-
topic: typing spec
Difficulty 2/5 1-3 hours Newbie friendliness 75/100
-
topic: documentation
Difficulty 2/5 1-3 hours Newbie friendliness 76/100
-
topic: documentation
Difficulty 2/5 1-3 hours Newbie friendliness 65/100
-
topic: conformance tests topic: typing spec
Difficulty 3/5 1-2 days Newbie friendliness 72/100
Similar issues
-
bug
Difficulty 2/5 1-3 hours Newbie friendliness 90/100
learningequality/ricecooker#747 ·
-
Difficulty 2/5 1-3 hours Newbie friendliness 68/100
BSData/horus-heresy-3rd-edition#3171 ·
-
enhancement
Difficulty 2/5 1-3 hours Newbie friendliness 72/100
-
Difficulty 2/5 1-3 hours Newbie friendliness 76/100
run-llama/llama_index#23199 ·
-
Difficulty 2/5 1-3 hours Newbie friendliness 84/100
KhronosGroup/glTF-Blender-IO#2769 ·