API for filtering / subsetting

[TomNicholas]

So far we've only really implemented dictionary-like get/setitem syntax, but we should add a variety of other ways to select nodes from a tree too. Here are some suggestions:

class DataTree:
    ...

    def __getitem__(self, key: str) -> DataTree | DataArray:
        """
        Accepts node/variable names, or file-like paths to nodes/variables (inc. '../var').

        (Also needs to accommodate indexing somehow.)
        """
        ...

    def subset(self, keys: Sequence[str]) -> DataTree:
        """
        Return new tree containing only nodes with names matching keys.

        (Could probably be combined with `__getitem__`. 
        Also unsure what the return type should be.)
        """
        ...

    @property
    def subtree(self) -> Iterator[DataTree]:
        """An iterator over all nodes in this tree, including both self and all descendants."""
        ...

    def filter(self, filterfunc: Callable) -> Iterator[DataTree]:
        """Filters subtree by returning only nodes for which `filterfunc(node)` is True."""
        ...

Are there other types of access that we're missing here? Filtering by regex match? Getting nodes where at least one part of the path matches ("tag-like" access)? Glob?

[TomNicholas]

@OriolAbril would these types of functions be sufficient for ArViz's usecases you think? From arviz-devs/arviz#2015 (comment):

dt[["posterior", "posterior_predictive"]] is not possible

getting a subset of the datatree that consists of multiple groups

This is what I'm suggesting subset do, or __getitem__.

applying a function to the variable x that is present in 3 out of 5 groups of the datatree.

I'm imagining enabling that via

dt.filter(lambda node: 'x' in node.variables).map_over_subtree(func)

Or we could potentially add an optional filterfunc argument to map_over_subtree.

[dcherian]

One possible point of (approximate) alignment with Xarray API is this issue: pydata/xarray#3894 for selecting using an iterable of variable names. This seems analogous to selecting nodes using subset

[TomNicholas]

I had not seen that issue, thanks @dcherian

[OriolAbril]

I think that would cover everything, but I'll try to think of examples so that we can also have things to test on.

We could also provide functions in datatree/xarray/arviz to act as filterfunc for common cases. My main question when thinking about using filter is storing the results back. I guess a merge would do it? With some renaming happening in the process maybe. It will probably be best to discuss with some examples.

[TomNicholas]

My main question when thinking about using filter is storing the results back.

Yes that's the tricky bit, because if you want to return a tree then you might need to retain nodes for which filterfunc(node)=False in order to still have a valid tree structure afterwards...

For example:

def name_is_lowercase(node)
   return node.name == node.name.lower() 

root = DataTree("a")
child = DataTree(parent=root, name="B")
grandchild = DataTree(parent=child, name="c") 

root.filter(name_is_lowercase)

This would return nodes "a" and "c", but it couldn't automatically reconstruct them into a tree without also preserving node "B".

If .filter just returned an iterator of nodes then you wouldn't need to be able to rebuild a tree, but this might not be most convenient for the user. This is why I would like to build these functions with some desired usage patterns in mind.

[TomNicholas]

I added a method to filter nodes based on some condition in #185