Switch LayerStackGetAttrKey to a custom dataclass type.

This avoids subclassing JAX's GetAttrKey, which will be changing
its implementation in the future.

penzai/core/tree_util.py

@@ -16,13 +16,24 @@
 
 from __future__ import annotations
 
+import dataclasses
 from typing import Any, Optional
 
 import jax
 
 PyTreeDef = jax.tree_util.PyTreeDef
 
 
+@dataclasses.dataclass(frozen=True)
+class CustomGetAttrKey:
+  """Subclass-friendly variant of jax.tree_util.GetAttrKey."""
+
+  name: str
+
+  def __str__(self):
+    return f".{self.name}"
+
+
 def tree_flatten_exactly_one_level(
     tree: Any,
 ) -> Optional[tuple[list[tuple[Any, Any]], PyTreeDef]]:
@@ -66,7 +77,10 @@ def pretty_keystr(keypath: tuple[Any, ...], tree: Any) -> str:
   parts = []
   for key in keypath:
     if isinstance(
-        key, jax.tree_util.GetAttrKey | jax.tree_util.FlattenedIndexKey
+        key,
+        jax.tree_util.GetAttrKey
+        | jax.tree_util.FlattenedIndexKey
+        | CustomGetAttrKey,
     ):
       parts.extend(("/", type(tree).__name__))
     split = tree_flatten_exactly_one_level(tree)

penzai/nn/layer_stack.py

@@ -17,10 +17,11 @@
 from __future__ import annotations
 
 import collections
+from collections.abc import Hashable
 import copy
 import dataclasses
 import enum
-from typing import Any, Callable, Hashable
+from typing import Any, Callable
 
 import jax
 from penzai.core import named_axes
@@ -39,7 +40,7 @@ class LayerStackVarBehavior(enum.Enum):
 
 
 @dataclasses.dataclass(frozen=True)
-class LayerStackGetAttrKey(jax.tree_util.GetAttrKey):
+class LayerStackGetAttrKey(pz_tree_util.CustomGetAttrKey):
   """GetAttrKey for LayerStack with extra metadata.
 
   This allows us to identify whether a given PyTree leaf is contained inside a

tests/nn/layer_stack_test.py

@@ -17,6 +17,7 @@
 from typing import Any
 from absl.testing import absltest
 import chex
+import collections
 import jax
 from penzai import pz
 
@@ -155,7 +156,18 @@ def builder(init_base_rng, some_value):
     unbound_layer, layer_vars = pz.unbind_variables(layer)
     unbound_slot_layer, slot_layer_vars = pz.unbind_variables(slot_layer)
 
-    chex.assert_trees_all_equal(unbound_layer, unbound_slot_layer)
+    # Check as dictionaries to avoid limitations of chex:
+    unbound_layer_leaves, unbound_layer_treedef = (
+        jax.tree_util.tree_flatten_with_path(unbound_layer)
+    )
+    unbound_slot_layer_leaves, unbound_slot_layer_treedef = (
+        jax.tree_util.tree_flatten_with_path(unbound_slot_layer)
+    )
+    self.assertEqual(unbound_layer_treedef, unbound_slot_layer_treedef)
+    chex.assert_trees_all_equal(
+        collections.OrderedDict(unbound_layer_leaves),
+        collections.OrderedDict(unbound_slot_layer_leaves),
+    )
 
     slot_layer_vars_dict = {var.label: var for var in slot_layer_vars}
     for var in layer_vars: