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2195718932 I_kwDOAMm_X86C4AMU 8855 DatasetView class breaks Liskov's rule. owenlittlejohns 7788154 open 0     2 2024-03-19T18:38:30Z 2024-03-19T19:13:17Z   CONTRIBUTOR      

What is your issue?

Working on migrating the datatree.py module into xarray/core revealed that the DatasetView class, which implements Dataset while disabling methods to mutate the object, breaks Liskov's substitution principle. The type for one of the overloads of DatasetView.__getitem__ is more general than the corresponding Dataset.__getitem__ signature (due to the use of Self in the Dataset signature).

```

In Dataset:

class Dataset(...): ... @overload def getitem(self, key: Iterable[Hashable]) -> Self: ... ```

The use of Self means that signature inherited from the superclass has a return type of DatasetView, but the DatasetView signature is overridden to have a return type of Dataset (the more generalised parent).

To avoid this, a couple of implementations were attempted:

  • A class that tries to intercept the methods that mutate the Dataset using getattr. This does not catch the __setitem__ method, as it is a Magic Method, and those aren't affected by getattr.
  • A Metaclass that can intercept Magic Methods, too. Implementation was inspired from here. I didn't get it to fully work, and eventually realised this was getting too complicated given the scope of the original problem.
  • A mix-in for the mutating methods. I couldn't get this to work in the timescale agreed upon.
  • Resorting back to ignoring the mypy errors for now, so we can proceed with the migration (given that there isn't a significant implementation concern identified from these type issues).

Also note, there is a tangentially-related known mypy error when a property setter accepts an argument of a different type to the property itself (https://github.com/python/mypy/issues/3004). This affects the assignment of Dataset objects to the DataTree.ds property. (Separate issue, but related)

 
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     xarray 13221727 issue

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