Setting var._attrs = None creates an empty dictionary?

[allen-adastra]

What is your issue?

If I do the following in repl, I get the weird behavior (described in comments).

>> import xarray as xr
>> import numpy as np
>> foo=xr.Variable(('x',), np.ones(10))
>> foo._attrs = None
>> print(foo._attrs) # Gives a None
>> foo
>> print(foo._attrs) # Gives a {}

Wondering what might be the cause of this? I know I am not using it as intended; this is for the xarray_jax experiment.

@shoyer would be good to get your perspective

[welcome]

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[keewis]

Variable (or rather, its base class NamedArray) uses xarray.core.formatting.array_repr to format the array, which accesses the attrs property to format the attributes:

xarray/xarray/namedarray/core.py

Lines 524 to 529 in 7bdc6d4

	@property
	def attrs(self) -> dict[Any, Any]:
	"""Dictionary of local attributes on this NamedArray."""
	if self._attrs is None:
	self._attrs = {}
	return self._attrs

Since _attrs is None at that point, it is replaced by an empty dict.

[allen-adastra]

Gotcha, if this is expected behavior, then perhaps in the context of xarray_jax the following is the solution

def _flatten_variable(
    v: xarray.Variable,
) -> Tuple[Tuple[jax.typing.ArrayLike], Tuple[Hashable, ...]]:
    children = (v._data,)
    aux = (
        v._dims,
        # Xarray will sometimes turn None into empty dictionaries. To maintain consistent tree structures, we convert None to empty dictionaries.
        # https://github.com/pydata/xarray/issues/9560
        {} if v._attrs is None else v._attrs,
    )
    return children, aux

[keewis]

also, interestingly, if you look at the setter you'll notice that we're storing None instead of an empty dict:

xarray/xarray/namedarray/core.py

Lines 531 to 533 in 7bdc6d4

	@attrs.setter
	def attrs(self, value: Mapping[Any, Any]) -> None:
	self._attrs = dict(value) if value else None

[shoyer]

aux = (

I would suggest using public APIs to side-step Xarray's caching layer is possible, e.g., v.data, v.dims, v.attrs.

[allen-adastra]

aux = (

I would suggest using public APIs to side-step Xarray's caching layer is possible, e.g., v.data, v.dims, v.attrs.

Nice, seems to work for .attrs. I've made a list of the places where the public API works and does not work on flattening (seems to be .data, .coords and .indexes that cause problems) allen-adastra/xarray_jax#14