issue_comments

+1 to shoyer - encapsulation is by far the easiest approach.

Why can't you initialise whatever you need to upon first access?

How do you do that?

```python @xarray.register_dataset_accessor('foo') class FooAccessor: def init(self, xarray_obj): self.obj = xarray_obj # <insert whatever health checks on xarray_obj> self.x = something(xarray_obj)

@property
def bar(self):
    # snip

def baz(self):
    # snip

```

The accessor __init__ method will be invoked by xarray the first time you invoke array.foo.

Note, however, that it is not recommended to put expensive calculations in it, because the object will possibly be destroyed and reinitialised every time the array is transformed - or in other words, anything that is not a read-only method/property or a cell update.

I emphasized possibly because some transforms may not destroy and recreate your accessor instance, thus potentially causing it to be in a state that is incoherent with the attached xarray object. Every time you invoke a method, you should verify that whatever assumptions you relied upon to generate the state are still valid.

What about simply wrapping xarray objects internally in your classes, along with utilities for converting to/from xarray objects? When users want to do domain specific stuff they can use your objects, and when they want to do something generic with xarray they can convert by calling a method like to_xarray.

Give a careful read to #3268 if statefulness (beyond what can be stored in xarray variables) is important for you.

I would not advise subclassing. In most cases it should work but it's thoroughly untested and I would be unsurprised if you were to find a use case that accidentally reverts your subclass to the base class.

I don't think I fully understand your problem with accessors. Is it because you would need a hook that is triggered by DataArray. __init__? Why can't you initialise whatever you need to upon first access?