Add option to prune variables after do intervention

pymc_experimental/model_transform/basic.py

@@ -0,0 +1,35 @@
+from pymc import Model
+from pytensor.graph import ancestors
+
+from pymc_experimental.utils.model_fgraph import (
+    ModelObservedRV,
+    ModelVar,
+    fgraph_from_model,
+    model_from_fgraph,
+)
+
+
+def prune_vars_detached_from_observed(model: Model) -> Model:
+    """Prune model variables that are not related to any observed variable in the Model."""
+
+    # Potentials are ambiguous as whether they correspond to likelihood or prior terms,
+    # We simply raise for now
+    if model.potentials:
+        raise NotImplementedError("Pruning not implemented for models with Potentials")
+
+    fgraph, _ = fgraph_from_model(model, inlined_views=True)
+    observed_vars = (
+        out
+        for node in fgraph.apply_nodes
+        if isinstance(node.op, ModelObservedRV)
+        for out in node.outputs
+    )
+    ancestor_nodes = {var.owner for var in ancestors(observed_vars)}
+    nodes_to_remove = {
+        node
+        for node in fgraph.apply_nodes
+        if isinstance(node.op, ModelVar) and node not in ancestor_nodes
+    }
+    for node_to_remove in nodes_to_remove:
+        fgraph.remove_node(node_to_remove)
+    return model_from_fgraph(fgraph)

pymc_experimental/model_transform/conditioning.py

@@ -4,6 +4,7 @@
 from pymc.pytensorf import _replace_vars_in_graphs
 from pytensor.tensor import TensorVariable
 
+from pymc_experimental.model_transform.basic import prune_vars_detached_from_observed
 from pymc_experimental.utils.model_fgraph import (
     ModelDeterministic,
     ModelFreeRV,
@@ -113,7 +114,9 @@ def replacement_fn(var, inner_replacements):
     return replaced_graphs
 
 
-def do(model: Model, vars_to_interventions: Dict[Union["str", TensorVariable], Any]) -> Model:
+def do(
+    model: Model, vars_to_interventions: Dict[Union["str", TensorVariable], Any], prune_vars=False
+) -> Model:
     """Replace model variables by intervention variables.
 
     Intervention variables will either show up as `Data` or `Deterministics` in the new model,
@@ -126,6 +129,9 @@ def do(model: Model, vars_to_interventions: Dict[Union["str", TensorVariable], A
         Dictionary that maps model variables (or names) to intervention expressions.
         Intervention expressions must have a shape and data type that is compatible
         with the original model variable.
+    prune_vars: bool, defaults to False
+        Whether to prune model variables that are not connected to any observed variables,
+        after the interventions.
 
     Returns
     -------
@@ -196,4 +202,7 @@ def do(model: Model, vars_to_interventions: Dict[Union["str", TensorVariable], A
     # Replace variables by interventions
     toposort_replace(fgraph, tuple(replacements.items()))
 
-    return model_from_fgraph(fgraph)
+    model = model_from_fgraph(fgraph)
+    if prune_vars:
+        return prune_vars_detached_from_observed(model)
+    return model

pymc_experimental/tests/model_transform/test_conditioning.py

@@ -192,3 +192,30 @@ def test_do_dims():
         },
     )
     assert do_m.named_vars_to_dims["y"] == ["test_dim"]
+
+
+@pytest.mark.parametrize("prune", (False, True))
+def test_do_prune(prune):
+
+    with pm.Model() as m:
+        x0 = pm.ConstantData("x0", 0)
+        x1 = pm.ConstantData("x1", 0)
+        y = pm.Normal("y")
+        y_det = pm.Deterministic("y_det", y + x0)
+        z = pm.Normal("z", y_det)
+        llike = pm.Normal("llike", z + x1, observed=0)
+
+    orig_named_vars = {"x0", "x1", "y", "y_det", "z", "llike"}
+    assert set(m.named_vars) == orig_named_vars
+
+    do_m = do(m, {y_det: x0 + 5}, prune_vars=prune)
+    if prune:
+        assert set(do_m.named_vars) == {"x0", "x1", "y_det", "z", "llike"}
+    else:
+        assert set(do_m.named_vars) == orig_named_vars
+
+    do_m = do(m, {z: 0.5}, prune_vars=prune)
+    if prune:
+        assert set(do_m.named_vars) == {"x1", "z", "llike"}
+    else:
+        assert set(do_m.named_vars) == orig_named_vars