issue_comments

Ok thanks for the recommandations.

I'm thinking of using this kind of class to handle object creation:

```python import xarray as xr import motion

class Analogs: def new(cls, ...): array = xr.DataArray(...) cls.verify(array) return array

@staticmethod
def verify(array):
    print("verification...")

@classmethod
def from_csv(cls, filename:str, ...):
    array = xr.DataArray(filename, ...)
    cls.verify(array)
    return array

```

Let's say I define another Markers class for another type of data. Is it possible that my accessor exposes different functions according to class e, g:

```python @xr.register_dataarray_accessor("foo") class DataArrayAccessor(object): def init(self, xarray_obj: xr.DataArray): self._obj = xarray_obj

def to_csv(self):
    """
    This function is exposed to all objects
    but behave differently if it is a Markers or an Analogs
    """

def marker_only_function(self):
    """
    This function is only exposed to 
    array created with the Marker class
    """

def analog_only_function(self):
    """
    This function is only exposed to
    array created with the Analog class
    """

```

My ultimate goal is to have this kind of API:

python analogs = ( Analogs.from_csv("filename.csv") .foo.center() # foo accessor generic method .foo.rectify() # foo accessor specific method for Analogs object .mean() # xarray method )

+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.

Is it because you would need a hook that is triggered by DataArray. init

Yes, I need to test some properties when creating the DataArray such as shape and dimensions properties.

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

How do you do that?

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?