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The filter function allows you to select a subtree by filtering based on a kind, all leaves whose field kind is a subclass of such type are kept, the rest are set to a special Nothing value.

class MyTree(to.Tree):
    a: int = to.field(node=True, kind=Parameter)
    b: int = to.field(node=True, kind=BatchStat)

tree = MyTree(a=1, b=2)

to.filter(tree, Parameter) # MyTree(a=1, b=Nothing)
to.filter(tree, BatchStat) # MyTree(a=Nothing, b=2)
Since Nothing is an empty Pytree it gets ignored by tree operations, this effectively allows you to easily operate on a subset of the fields:

jax.tree_map(lambda x: -x, to.filter(tree, Parameter)) # MyTree(a=-1, b=Nothing)
jax.tree_map(lambda x: -x, to.filter(tree, BatchStat)) # MyTree(a=Nothing, b=-2)

filter predicates

If you need to do more complex filtering, you can pass callables with the signature

FieldInfo -> bool

instead of types:

# all Parameters whose field name is "kernel"
    lambda field: issubclass(field.kind, Parameter) 
    and == "kernel"
# MyTree(a=Nothing, b=Nothing)

Since filter works with pytrees in general, the following is possible:

def array_like(field):
    return hasattr(field.value, "shape") and hasattr(field.value, "dtype")

tree = [1, np.2, jnp.array([3.0, 4.0])]

to.filter(tree, array_like) # [Nothing, np.2, jnp.array([3.0, 4.0])]

multiple filters

You can some queries by using multiple filters as *args. For a field to be kept it will required that all filters pass. Since passing types by themselves are "kind filters", one of the previous examples could be written as:

# all Parameters whose field name is "kernel"
    lambda field: == "kernel"
# MyTree(a=Nothing, b=Nothing)


If inplace is True, the input obj is mutated and returned. You can only update inplace if the input obj has a __dict__ attribute, else a TypeError is raised.