How to use typing.Never to indicate a subclass method with zero arguments cannot be called?
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Description
Feature
If you have a subclass that has a method defined in the superclass, but in the subclass should never get called, you should be able to declare self to have type Never.
Pitch
Let's suppose I have a base class Base with a method foo(), and a subclass Sub that raises an Exception if foo() is called. I'd like it so that the type checker flags an error on Sub.foo(), but not Base.foo(), so I tried this:
from typing_extensions import Never
class Base:
def __init__(self):
pass
def foo(self) -> None:
print("foo")
def goo(self, x: int) -> None;
print("goo ", x)
class Sub(Base):
def foo(self: Never) -> None:
raise Exception("never")
def goo(self, x: Never) -> None:
raise Exception("never")
x = Sub()
x.foo()
x.goo(3)
With mypy 1.0.0 and python 3.10, I get the following:
neversub.py:16: error: The erased type of self "<nothing>" is not a supertype of its class "neversub.Sub" [misc]
neversub.py:19: error: Argument 1 of "goo" is incompatible with supertype "Base"; supertype defines the argument type as "int" [override]
neversub.py:19: note: This violates the Liskov substitution principle
neversub.py:19: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
neversub.py:24: error: Invalid self argument "Sub" to attribute function "foo" with type "Callable[[NoReturn], None]" [misc]
neversub.py:25: error: Argument 1 to "goo" of "Sub" has incompatible type "int"; expected "NoReturn" [arg-type]
The last two errors are exactly what I want. But while the first error makes sense, aside from using a # type: ignore, I'm not sure how this should be declared to avoid having to use # type: ignore. In other words, what is the right way to indicate that a method with zero parameters is not valid to call in a subclass? Secondly, shouldn't the declaration of goo be considered as not violating the Liskov substitution principle based on how typing.Never is supposed to work?
I should note that with the method goo(), since it has a required argument, I can use the Never declaration to show that this is invalid. What's not clear is how do you indicate that a method with no arguments is invalid without having to use # type: ignore ?
I think this is a valid use of Never, and Eric Traut from pyright agrees: https://github.com/microsoft/pyright/issues/4653#issuecomment-1434851320
Could mypy do the same?
Contributor guide
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.
Research direction
Start by reproducing the provided Base/Sub example with mypy 1.0.0 and compare the diagnostics for foo and goo with the intended Never behavior. The work is done when a zero-argument subclass method can be declared as uncallable without a type: ignore and the goo override is handled consistently with the Liskov substitution rule.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- compilers
- Issue type
- Feature
- Difficulty
- 5/5
- Estimated time
- Over a week
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 30/100