Add abstractmethods to backend classes

[Illviljan]

It's been unclear to me what methods are necessary to implement or not. I think decorating with @abstractmethod will help with that. It's a breaking change though and it could be disruptive.

• Closes #xxxx
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[TomNicholas]

This seems like a good idea to me, but I don't know much about this part of the codebase. We should at minimum state in the docstring of these method whether they are required or optional.

From the errors thrown in the tests it seems set_variable is not defined by backends quite regularly at least.

[jhamman]

Thanks for working on this. Two small comments.

[jhamman]

Per #7437 (comment), we may not want to require all of these methods. We may end up in a situation where BackendEntrypoints must define one or more of open_dataarray, open_dataset, and open_datatree.

cc @jthielen, @keewis

[TomNicholas]

This seems pretty doable, e.g.

classMyBaseClass:
def__init__(self):
super().__init__()
# Check that at least one of the methods has been overriddenifall(getattr(self.__class__, m) isgetattr(MyBaseClass, m) formin ['method1', 'method2', 'method3']):
raiseTypeError("You must implement at least one of 'method1', 'method2', 'method3'")
defmethod1(self):
passdefmethod2(self):
passdefmethod3(self):
passclassMyDerivedClass(MyBaseClass):
defmethod1(self):
print("Method1 implemented")
# This will not raise an errord=MyDerivedClass()
classMyInvalidDerivedClass(MyBaseClass):
pass# This will raise a TypeErrori=MyInvalidDerivedClass()

TypeError: You must implement at least one of 'method1', 'method2', 'method3'