Subclass class variables not checked against superclass type hints

[jli]

Environment data

• Language Server version: 2021.8.2 (pyright f462f5bb)
• OS and version: Mac Big Sur 11.5.2
• Python version: Python 3.9.4, mypy 0.812

Expected v. actual behaviour

When a subclass sets a class variable that doesn't match the superclass's type hint, pylance should warn about it.

Pylance also infers the wrong type for subclass instance references to the variable.

Code Snippet / Additional information

class Experiment:
    train_epochs: int

    def __init__(self, train_epochs: int):
        self.train_epochs = train_epochs

class ExperimentWithPresets(Experiment):
    # different type than the superclass.
    # hovering over this, Pylance says `str`, which is correct
    train_epochs = "123"

    def __init__(self):
        pass

exp = Experiment(100)
print(f'base Experiment: {exp.train_epochs=}, {type(exp.train_epochs)=}')

# when hovering over exp2.train_epochs, Pylance says `int`, which is wrong
exp2 = ExperimentWithPresets()
print(f'ExperimentWithPresets: {exp2.train_epochs=}, {type(exp2.train_epochs)=}')

Output from python:

% python subclass_fields_type_mismatch.py
base Experiment: exp.train_epochs=100, type(exp.train_epochs)=<class 'int'>
ExperimentWithPresets: exp2.train_epochs='123', type(exp2.train_epochs)=<class 'str'>

Output from mypy:

% mypy subclass_fields_type_mismatch.py
subclass_fields_type_mismatch.py:10: error: Incompatible types in assignment (expression has type "str", base class "Experiment" defined the type as "int")
Found 1 error in 1 file (checked 1 source file)

[erictraut]

Thanks for reporting this. I agree this is an omission in type checking. Pylance does check the base class type for instance variable assignments, but it doesn't currently do so for class variable assignments in subclasses.

[erictraut]

This will be addressed in the next release.

[jli]

🙏 thanks so much, really appreciate your responsiveness!

[jakebailey]

This issue has been fixed in version 2021.7.0, which we've just released. You can find the changelog here: https://github.com/microsoft/pylance-release/blob/main/CHANGELOG.md#202190-1-september-2021