Migrate iterators.py for datatree. (pydata#8879)

* Migrate iterators.py for datatree.

* Add __future__.annotations for Python 3.9.

* Fix documentation typo in GitHub URL.

* Improve type hints and documentation strings.

* Fix DataTree docstring examples.

* Add anytree license.

* DAS-2063: Changes to use just LevelOrderIter

* Minor whitespace tweaks.

---------

Co-authored-by: Matt Savoie <[email protected]>

Author: owenlittlejohns

Diff for: doc/whats-new.rst

@@ -113,6 +113,9 @@ Internal Changes
 - Migrates ``datatree`` functionality into ``xarray/core``. (:pull: `8789`)
   By `Owen Littlejohns <https://github.com/owenlittlejohns>`_, `Matt Savoie
   <https://github.com/flamingbear>`_ and `Tom Nicholas <https://github.com/TomNicholas>`_.
+- Migrates ``iterator`` functionality into ``xarray/core`` (:pull: `8879`)
+  By `Owen Littlejohns <https://github.com/owenlittlejohns>`_, `Matt Savoie
+  <https://github.com/flamingbear>`_ and `Tom Nicholas <https://github.com/TomNicholas>`_.
 
 
 .. _whats-new.2024.02.0:

Diff for: licenses/ANYTREE_LICENSE

@@ -0,0 +1,201 @@
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Diff for: xarray/core/iterators.py

@@ -0,0 +1,131 @@
+from __future__ import annotations
+
+from collections import abc
+from collections.abc import Iterator
+from typing import Callable
+
+from xarray.core.treenode import Tree
+
+"""These iterators are copied from anytree.iterators, with minor modifications."""
+
+
+class LevelOrderIter(abc.Iterator):
+    """Iterate over tree applying level-order strategy starting at `node`.
+    This is the iterator used by `DataTree` to traverse nodes.
+
+    Parameters
+    ----------
+    node : Tree
+        Node in a tree to begin iteration at.
+    filter_ : Callable, optional
+        Function called with every `node` as argument, `node` is returned if `True`.
+        Default is to iterate through all ``node`` objects in the tree.
+    stop : Callable, optional
+        Function that will cause iteration to stop if ``stop`` returns ``True``
+        for ``node``.
+    maxlevel : int, optional
+        Maximum level to descend in the node hierarchy.
+
+    Examples
+    --------
+    >>> from xarray.core.datatree import DataTree
+    >>> from xarray.core.iterators import LevelOrderIter
+    >>> f = DataTree(name="f")
+    >>> b = DataTree(name="b", parent=f)
+    >>> a = DataTree(name="a", parent=b)
+    >>> d = DataTree(name="d", parent=b)
+    >>> c = DataTree(name="c", parent=d)
+    >>> e = DataTree(name="e", parent=d)
+    >>> g = DataTree(name="g", parent=f)
+    >>> i = DataTree(name="i", parent=g)
+    >>> h = DataTree(name="h", parent=i)
+    >>> print(f)
+    DataTree('f', parent=None)
+    ├── DataTree('b')
+    │   ├── DataTree('a')
+    │   └── DataTree('d')
+    │       ├── DataTree('c')
+    │       └── DataTree('e')
+    └── DataTree('g')
+        └── DataTree('i')
+            └── DataTree('h')
+    >>> [node.name for node in LevelOrderIter(f)]
+    ['f', 'b', 'g', 'a', 'd', 'i', 'c', 'e', 'h']
+    >>> [node.name for node in LevelOrderIter(f, maxlevel=3)]
+    ['f', 'b', 'g', 'a', 'd', 'i']
+    >>> [
+    ...     node.name
+    ...     for node in LevelOrderIter(f, filter_=lambda n: n.name not in ("e", "g"))
+    ... ]
+    ['f', 'b', 'a', 'd', 'i', 'c', 'h']
+    >>> [node.name for node in LevelOrderIter(f, stop=lambda n: n.name == "d")]
+    ['f', 'b', 'g', 'a', 'i', 'h']
+    """
+
+    def __init__(
+        self,
+        node: Tree,
+        filter_: Callable | None = None,
+        stop: Callable | None = None,
+        maxlevel: int | None = None,
+    ):
+        self.node = node
+        self.filter_ = filter_
+        self.stop = stop
+        self.maxlevel = maxlevel
+        self.__iter = None
+
+    def __init(self):
+        node = self.node
+        maxlevel = self.maxlevel
+        filter_ = self.filter_ or LevelOrderIter.__default_filter
+        stop = self.stop or LevelOrderIter.__default_stop
+        children = (
+            []
+            if LevelOrderIter._abort_at_level(1, maxlevel)
+            else LevelOrderIter._get_children([node], stop)
+        )
+        return self._iter(children, filter_, stop, maxlevel)
+
+    @staticmethod
+    def __default_filter(node: Tree) -> bool:
+        return True
+
+    @staticmethod
+    def __default_stop(node: Tree) -> bool:
+        return False
+
+    def __iter__(self) -> Iterator[Tree]:
+        return self
+
+    def __next__(self) -> Iterator[Tree]:
+        if self.__iter is None:
+            self.__iter = self.__init()
+        item = next(self.__iter)  # type: ignore[call-overload]
+        return item
+
+    @staticmethod
+    def _abort_at_level(level: int, maxlevel: int | None) -> bool:
+        return maxlevel is not None and level > maxlevel
+
+    @staticmethod
+    def _get_children(children: list[Tree], stop: Callable) -> list[Tree]:
+        return [child for child in children if not stop(child)]
+
+    @staticmethod
+    def _iter(
+        children: list[Tree], filter_: Callable, stop: Callable, maxlevel: int | None
+    ) -> Iterator[Tree]:
+        level = 1
+        while children:
+            next_children = []
+            for child in children:
+                if filter_(child):
+                    yield child
+                next_children += LevelOrderIter._get_children(
+                    list(child.children.values()), stop
+                )
+            children = next_children
+            level += 1
+            if LevelOrderIter._abort_at_level(level, maxlevel):
+                break

Diff for: xarray/core/treenode.py

@@ -333,9 +333,9 @@ def subtree(self: Tree) -> Iterator[Tree]:
         --------
         DataTree.descendants
         """
-        from xarray.datatree_.datatree import iterators
+        from xarray.core.iterators import LevelOrderIter
 
-        return iterators.PreOrderIter(self)
+        return LevelOrderIter(self)
 
     @property
     def descendants(self: Tree) -> tuple[Tree, ...]: