Do not rely on model variable ordering for initival replacements

It is not part of the API that variables must be registered in topological order

Author: ricardoV94

pymc/initial_point.py

@@ -25,7 +25,13 @@
 from pytensor.tensor.variable import TensorVariable
 
 from pymc.logprob.transforms import Transform
-from pymc.pytensorf import compile_pymc, find_rng_nodes, replace_rng_nodes, reseed_rngs
+from pymc.pytensorf import (
+    compile_pymc,
+    find_rng_nodes,
+    replace_rng_nodes,
+    reseed_rngs,
+    toposort_replace,
+)
 from pymc.util import get_transformed_name, get_untransformed_name, is_transformed_name
 
 StartDict = dict[Variable | str, np.ndarray | Variable | str]
@@ -288,8 +294,7 @@ def make_initial_point_expression(
     # order, so that later nodes do not reintroduce expressions with earlier
     # rvs that would need to once again be replaced by their initial_points
     graph = FunctionGraph(outputs=free_rvs_clone, clone=False)
-    replacements = reversed(list(zip(free_rvs_clone, initial_values_clone)))
-    graph.replace_all(replacements, import_missing=True)
+    toposort_replace(graph, tuple(zip(free_rvs_clone, initial_values_clone)), reverse=True)
 
     if not return_transformed:
         return graph.outputs

tests/test_initial_point.py

@@ -47,12 +47,17 @@ def test_make_initial_point_fns_per_chain_checks_kwargs(self):
             )
         pass
 
-    def test_dependent_initvals(self):
+    @pytest.mark.parametrize("reverse_rvs", [False, True])
+    def test_dependent_initvals(self, reverse_rvs):
         with pm.Model() as pmodel:
             L = pm.Uniform("L", 0, 1, initval=0.5)
             U = pm.Uniform("U", lower=9, upper=10, initval=9.5)
             B1 = pm.Uniform("B1", lower=L, upper=U, initval=5)
             B2 = pm.Uniform("B2", lower=L, upper=U, initval=(L + U) / 2)
+
+            if reverse_rvs:
+                pmodel.free_RVs = pmodel.free_RVs[::-1]
+
             ip = pmodel.initial_point(random_seed=0)
             assert ip["L_interval__"] == 0
             assert ip["U_interval__"] == 0