Improve collect_default_updates

* It works with nested RNGs
* It raises error if RNG used in SymbolicRandomVariable is not given an update
* It raises warning if same RNG is used in multiple nodes

pymc/distributions/distribution.py

@@ -593,7 +593,9 @@ class CustomSymbolicDistRV(SymbolicRandomVariable):
 
     def update(self, node: Node):
         op = node.op
-        inner_updates = collect_default_updates(op.inner_inputs, op.inner_outputs)
+        inner_updates = collect_default_updates(
+            op.inner_inputs, op.inner_outputs, must_be_shared=False
+        )
 
         # Map inner updates to outer inputs/outputs
         updates = {}

pymc/pytensorf.py

@@ -42,7 +42,6 @@
     Variable,
     clone_get_equiv,
     graph_inputs,
-    vars_between,
     walk,
 )
 from pytensor.graph.fg import FunctionGraph
@@ -51,6 +50,7 @@
 from pytensor.tensor.basic import _as_tensor_variable
 from pytensor.tensor.elemwise import Elemwise
 from pytensor.tensor.random.op import RandomVariable
+from pytensor.tensor.random.type import RandomType
 from pytensor.tensor.random.var import (
     RandomGeneratorSharedVariable,
     RandomStateSharedVariable,
@@ -1000,42 +1000,85 @@ def reseed_rngs(
 
 
 def collect_default_updates(
-    inputs: Sequence[Variable], outputs: Sequence[Variable]
+    inputs: Sequence[Variable],
+    outputs: Sequence[Variable],
+    must_be_shared: bool = True,
 ) -> Dict[Variable, Variable]:
-    """Collect default update expression of RVs between inputs and outputs"""
+    """Collect default update expression for shared-variable RNGs used by RVs between inputs and outputs.
+
+    If `must_be_shared` is False, update expressions will also be returned for non-shared input RNGs.
+    This can be useful to obtain the symbolic update expressions from inner graphs.
+    """
 
     # Avoid circular import
     from pymc.distributions.distribution import SymbolicRandomVariable
 
+    def find_default_update(clients, rng: Variable) -> Union[None, Variable]:
+        rng_clients = clients.get(rng, None)
+
+        # Root case, RNG is not used elsewhere
+        if not rng_clients:
+            return rng
+
+        if len(rng_clients) > 1:
+            warnings.warn(
+                f"RNG Variable {rng} has multiple clients. This is likely an inconsistent random graph.",
+                UserWarning,
+            )
+            return None
+
+        [client, _] = rng_clients[0]
+
+        # RNG is an output of the function, this is not a problem
+        if client == "output":
+            return rng
+
+        # RNG is used by another operator, which should output an update for the RNG
+        if isinstance(client.op, RandomVariable):
+            # RandomVariable first output is always the update of the input RNG
+            next_rng = client.outputs[0]
+
+        elif isinstance(client.op, SymbolicRandomVariable):
+            # SymbolicRandomVariable have an explicit method that returns an
+            # update mapping for their RNG(s)
+            next_rng = client.op.update(client).get(rng)
+            if next_rng is None:
+                raise ValueError(
+                    f"No update mapping found for RNG used in SymbolicRandomVariable Op {client.op}"
+                )
+        else:
+            # We don't know how this RNG should be updated (e.g., Scan).
+            # The user should provide an update manually
+            return None
+
+        # Recurse until we find final update for RNG
+        return find_default_update(clients, next_rng)
+
+    outputs = makeiter(outputs)
+    fg = FunctionGraph(outputs=outputs, clone=False)
+    clients = fg.clients
+
     rng_updates = {}
-    output_to_list = outputs if isinstance(outputs, (list, tuple)) else [outputs]
-    for random_var in (
-        var
-        for var in vars_between(inputs, output_to_list)
-        if var.owner
-        and isinstance(var.owner.op, (RandomVariable, SymbolicRandomVariable))
-        and var not in inputs
+    # Iterate over input RNGs. Only consider shared RNGs if `must_be_shared==True`
+    for input_rng in (
+        inp
+        for inp in graph_inputs(outputs, blockers=inputs)
+        if (
+            (not must_be_shared or isinstance(inp, SharedVariable))
+            and isinstance(inp.type, RandomType)
+        )
     ):
-        # All nodes in `vars_between(inputs, outputs)` have owners.
-        # But mypy doesn't know, so we just assert it:
-        assert random_var.owner.op is not None
-        if isinstance(random_var.owner.op, RandomVariable):
-            rng = random_var.owner.inputs[0]
-            if getattr(rng, "default_update", None) is not None:
-                update_map = {rng: rng.default_update}
-            else:
-                update_map = {rng: random_var.owner.outputs[0]}
+        # Even if an explicit default update is provided, we call it to
+        # issue any warnings about invalid random graphs.
+        default_update = find_default_update(clients, input_rng)
+
+        # Respect default update if provided
+        if getattr(input_rng, "default_update", None):
+            rng_updates[input_rng] = input_rng.default_update
         else:
-            update_map = random_var.owner.op.update(random_var.owner)
-        # Check that we are not setting different update expressions for the same variables
-        for rng, update in update_map.items():
-            if rng not in rng_updates:
-                rng_updates[rng] = update
-            # When a variable has multiple outputs, it will be called twice with the same
-            # update expression. We don't want to raise in that case, only if the update
-            # expression in different from the one already registered
-            elif rng_updates[rng] is not update:
-                raise ValueError(f"Multiple update expressions found for the variable {rng}")
+            if default_update is not None:
+                rng_updates[input_rng] = default_update
+
     return rng_updates
 
 

tests/test_pytensorf.py

@@ -11,6 +11,8 @@
 #   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
+import warnings
+
 from unittest import mock
 
 import numpy as np
@@ -38,6 +40,7 @@
 from pymc.exceptions import NotConstantValueError
 from pymc.logprob.utils import ParameterValueError
 from pymc.pytensorf import (
+    collect_default_updates,
     compile_pymc,
     constant_fold,
     convert_observed_data,
@@ -406,28 +409,63 @@ def test_compile_pymc_updates_inputs(self):
             # Each RV adds a shared output for its rng
             assert len(fn_fgraph.outputs) == 1 + rvs_in_graph
 
-    # Disable `reseed_rngs` so that we can test with simpler update rule
-    @mock.patch("pymc.pytensorf.reseed_rngs")
-    def test_compile_pymc_custom_update_op(self, _):
-        """Test that custom MeasurableVariable Op updates are used by compile_pymc"""
+    def test_compile_pymc_symbolic_rv_update(self):
+        """Test that SymbolicRandomVariable Op update methods are used by compile_pymc"""
 
         class NonSymbolicRV(OpFromGraph):
             def update(self, node):
-                return {node.inputs[0]: node.inputs[0] + 1}
+                return {node.inputs[0]: node.outputs[0]}
 
-        dummy_inputs = [pt.scalar(), pt.scalar()]
-        dummy_outputs = [pt.add(*dummy_inputs)]
-        dummy_x = NonSymbolicRV(dummy_inputs, dummy_outputs)(pytensor.shared(1.0), 1.0)
+        rng = pytensor.shared(np.random.default_rng())
+        dummy_rng = rng.type()
+        dummy_next_rng, dummy_x = NonSymbolicRV(
+            [dummy_rng], pt.random.normal(rng=dummy_rng).owner.outputs
+        )(rng)
 
         # Check that there are no updates at first
         fn = compile_pymc(inputs=[], outputs=dummy_x)
-        assert fn() == fn() == 2.0
+        assert fn() == fn()
 
         # And they are enabled once the Op is registered as a SymbolicRV
         SymbolicRandomVariable.register(NonSymbolicRV)
-        fn = compile_pymc(inputs=[], outputs=dummy_x)
-        assert fn() == 2.0
-        assert fn() == 3.0
+        fn = compile_pymc(inputs=[], outputs=dummy_x, random_seed=431)
+        assert fn() != fn()
+
+    def test_compile_pymc_symbolic_rv_missing_update(self):
+        """Test that error is raised if SymbolicRandomVariable Op does not
+        provide rule for updating RNG"""
+
+        class SymbolicRV(OpFromGraph):
+            def update(self, node):
+                # Update is provided for rng1 but not rng2
+                return {node.inputs[0]: node.outputs[0]}
+
+        SymbolicRandomVariable.register(SymbolicRV)
+
+        # No problems at first, as the one RNG is given the update rule
+        rng1 = pytensor.shared(np.random.default_rng())
+        dummy_rng1 = rng1.type()
+        dummy_next_rng1, dummy_x1 = SymbolicRV(
+            [dummy_rng1],
+            pt.random.normal(rng=dummy_rng1).owner.outputs,
+        )(rng1)
+        fn = compile_pymc(inputs=[], outputs=dummy_x1, random_seed=433)
+        assert fn() != fn()
+
+        # Now there's a problem as there is no update rule for rng2
+        rng2 = pytensor.shared(np.random.default_rng())
+        dummy_rng2 = rng2.type()
+        dummy_next_rng1, dummy_x1, dummy_next_rng2, dummy_x2 = SymbolicRV(
+            [dummy_rng1, dummy_rng2],
+            [
+                *pt.random.normal(rng=dummy_rng1).owner.outputs,
+                *pt.random.normal(rng=dummy_rng2).owner.outputs,
+            ],
+        )(rng1, rng2)
+        with pytest.raises(
+            ValueError, match="No update mapping found for RNG used in SymbolicRandomVariable"
+        ):
+            compile_pymc(inputs=[], outputs=[dummy_x1, dummy_x2])
 
     def test_random_seed(self):
         seedx = pytensor.shared(np.random.default_rng(1))
@@ -457,15 +495,62 @@ def test_random_seed(self):
         assert y3_eval == y2_eval
 
     def test_multiple_updates_same_variable(self):
-        rng = pytensor.shared(np.random.default_rng(), name="rng")
-        x = pt.random.normal(rng=rng)
-        y = pt.random.normal(rng=rng)
-
-        assert compile_pymc([], [x])
-        assert compile_pymc([], [y])
-        msg = "Multiple update expressions found for the variable rng"
-        with pytest.raises(ValueError, match=msg):
-            compile_pymc([], [x, y])
+        # Raise if unexpected warning is issued
+        with warnings.catch_warnings():
+            warnings.simplefilter("error")
+
+            rng = pytensor.shared(np.random.default_rng(), name="rng")
+            x = pt.random.normal(rng=rng)
+            y = pt.random.normal(rng=rng)
+
+            # No warnings if only one variable is used
+            assert compile_pymc([], [x])
+            assert compile_pymc([], [y])
+
+            user_warn_msg = "RNG Variable rng has multiple clients"
+            with pytest.warns(UserWarning, match=user_warn_msg):
+                f = compile_pymc([], [x, y], random_seed=456)
+            assert f() == f()
+
+            # The user can provide an explicit update, but we will still issue a warning
+            with pytest.warns(UserWarning, match=user_warn_msg):
+                f = compile_pymc([], [x, y], updates={rng: y.owner.outputs[0]}, random_seed=456)
+            assert f() != f()
+
+            # Same with default update
+            rng.default_update = x.owner.outputs[0]
+            with pytest.warns(UserWarning, match=user_warn_msg):
+                f = compile_pymc([], [x, y], updates={rng: y.owner.outputs[0]}, random_seed=456)
+            assert f() != f()
+
+    def test_nested_updates(self):
+        rng = pytensor.shared(np.random.default_rng())
+        next_rng1, x = pt.random.normal(rng=rng).owner.outputs
+        next_rng2, y = pt.random.normal(rng=next_rng1).owner.outputs
+        next_rng3, z = pt.random.normal(rng=next_rng2).owner.outputs
+
+        collect_default_updates([], [x, y, z]) == {rng: next_rng3}
+
+        fn = compile_pymc([], [x, y, z], random_seed=514)
+        assert not set(list(np.array(fn()))) & set(list(np.array(fn())))
+
+        # A local myopic rule (as PyMC used before, would not work properly)
+        fn = pytensor.function([], [x, y, z], updates={rng: next_rng1})
+        assert set(list(np.array(fn()))) & set(list(np.array(fn())))
+
+
+def test_collect_default_updates_must_be_shared():
+    shared_rng = pytensor.shared(np.random.default_rng())
+    nonshared_rng = shared_rng.type()
+
+    next_rng_of_shared, x = pt.random.normal(rng=shared_rng).owner.outputs
+    next_rng_of_nonshared, y = pt.random.normal(rng=nonshared_rng).owner.outputs
+
+    res = collect_default_updates(inputs=[nonshared_rng], outputs=[x, y])
+    assert res == {shared_rng: next_rng_of_shared}
+
+    res = collect_default_updates(inputs=[nonshared_rng], outputs=[x, y], must_be_shared=False)
+    assert res == {shared_rng: next_rng_of_shared, nonshared_rng: next_rng_of_nonshared}
 
 
 def test_replace_rng_nodes():