Inherited `ClassVar`

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Introduction

In Python, the ClassVar type hint was introduced to indicate that a class attribute is intended to be a class variable, rather than an instance variable. However, when it comes to inheritance, the behavior of ClassVar can be counterintuitive. In this article, we will delve into the issue of inherited ClassVar and explore potential improvements to the current implementation.

The Issue with Inherited ClassVar

Let's consider the following example:

from typing import ClassVar


class B:
    x: ClassVar[int | str] = 0


class C(B):
    x = "random"  # pyrefly reports error here "Class member `x` overrides parent class `B` in an inconsistent manner"

As we can see, the x attribute in class C is assigned a value of "random", which is a str. However, pyrefly reports an error, stating that the x attribute overrides the parent class B in an inconsistent manner. This seems counterintuitive, as the annotation of x in B explicitly allows it to be either an int or a str.

The Problem with Current Implementations

The current implementation of ClassVar in mypy and pyright has some limitations. Let's consider the following example:

from typing import ClassVar, reveal_type


class B:
    x: ClassVar[int | str] = 0


class C(B):
    x = "random"


class D(C):
    x = 1  # mypy complains where it shouldn't


class E(D):
    x = b"3"  # pyright doesn't complain where it should


def foo(b: type[B]) -> None:
    reveal_type(b.x)
    if isinstance(b.x, int):
        print(b.x + 8)
    else:
        print(b.x + "8")


foo(E)

In this example, the annotation of x in B should apply to all subclasses. However, mypy complains about the assignment of x in class D, which is an int. On the other hand, pyright doesn't complain about the assignment of x in class E, which is a bytes object.

Potential Improvements

To address the issue with inherited ClassVar, we need to improve the current implementation. Here are some potential improvements:

  1. Allow ClassVar to be overridden: The current implementation of ClassVar does not allow it to be overridden in subclasses. This can lead to unexpected behavior and errors. We should allow ClassVar to be overridden, but with some restrictions.
  2. Improve type checking: The current implementation of ClassVar has some limitations when it comes to type checking. We should improve the type checking to ensure that the annotation of x in B is respected in all subclasses.
  3. Provide better error messages: The current implementation of ClassVar provides error messages that are not very informative. We should provide better error messages that explain the issue and suggest a solution.

Conclusion

In conclusion, the issue with inherited ClassVar is a complex problem that requires a deep understanding of the current implementation and its limitations. By improving the current implementation, we can provide better support for ClassVar and make it easier for developers to use. We should allow ClassVar to be overridden, improve type checking, and provide better error messages.

Future Work

To further improve the implementation of ClassVar, we should consider the following:

  1. Add support for ClassVar in more type checkers: Currently, ClassVar is only supported in mypy and pyright. We should add support for ClassVar in more type checkers, such as pyrefly and pytype.
  2. Improve the documentation: The documentation for ClassVar is not very comprehensive. We should improve the documentation to provide more information about the usage and behavior of ClassVar.
  3. Provide more examples: The current implementation of ClassVar has some limitations when it comes to examples. We should provide more examples to demonstrate the usage and behavior of ClassVar.

Q: What is the issue with inherited ClassVar in Python?

A: The issue with inherited ClassVar in Python is that it does not allow subclasses to override the class attribute with a different type. This can lead to unexpected behavior and errors.

Q: Why is this an issue?

A: This is an issue because it can lead to unexpected behavior and errors. For example, if a subclass tries to override the class attribute with a different type, it may not be caught by the type checker, leading to runtime errors.

Q: How can I work around this issue?

A: There are a few ways to work around this issue:

  1. Use a different type hint: Instead of using ClassVar, you can use a different type hint, such as Final or Constant.
  2. Use a class attribute with a different name: Instead of using the same name for the class attribute in the subclass, you can use a different name.
  3. Use a metaclass: You can use a metaclass to define the class attribute in the subclass.

Q: Why is mypy complaining about my code?

A: Mypy is complaining about your code because it is trying to enforce the type hinting rules for ClassVar. If you are trying to override the class attribute with a different type, mypy may complain about it.

Q: How can I fix the error message from mypy?

A: To fix the error message from mypy, you can try one of the following:

  1. Change the type hint: Change the type hint for the class attribute to match the type of the value you are trying to assign.
  2. Use a different type hint: Use a different type hint, such as Final or Constant, instead of ClassVar.
  3. Use a class attribute with a different name: Use a different name for the class attribute in the subclass.

Q: Why is pyright not complaining about my code?

A: Pyright is not complaining about your code because it is not enforcing the type hinting rules for ClassVar as strictly as mypy. However, this does not mean that your code is correct, and you should still try to follow the type hinting rules for ClassVar.

Q: How can I get better error messages from pyright?

A: To get better error messages from pyright, you can try the following:

  1. Use the --strict flag: Use the --strict flag when running pyright to enable strict type checking.
  2. Use the --type-check flag: Use the --type-check flag when running pyright to enable type checking.
  3. Check the pyright documentation: Check the pyright documentation for more information on how to use pyright and get better error messages.

Q: What is the future of ClassVar in Python?

A: The future of ClassVar in Python is not clear. However, it is likely that the type hinting rules for ClassVar will be improved in future versions of Python. In the meantime, you can try to work around the issues with ClassVar by using different type hints or class attributes with different names.