copy_function_with_new_rngs warns with JAXLinker

pymc/pytensorf.py

@@ -38,6 +38,7 @@
 )
 from pytensor.graph.fg import FunctionGraph, Output
 from pytensor.graph.op import Op
+from pytensor.link.jax.linker import JAXLinker
 from pytensor.scalar.basic import Cast
 from pytensor.scan.op import Scan
 from pytensor.tensor.basic import _as_tensor_variable
@@ -1208,6 +1209,15 @@ def copy_function_with_new_rngs(
     fn_ = fn.f if isinstance(fn, PointFunc) else fn
     shared_rngs = [var for var in fn_.get_shared() if isinstance(var.type, RandomGeneratorType)]
     n_shared_rngs = len(shared_rngs)
+    if n_shared_rngs > 0 and isinstance(fn_.maker.linker, JAXLinker):
+        # Reseeding RVs in JAX backend requires a different logic, becuase the SharedVariables
+        # used internally are not the ones that `function.get_shared()` returns.
+        warnings.warn(
+            "At the moment, it is not possible to set the random generator's key for "
+            "JAX linked functions. This means that the draws yielded by the random "
+            "variables that are requested by 'Deterministic' will not be reproducible."
+        )
+        return fn
     swap = {
         old_shared_rng: shared(rng, borrow=True)
         for old_shared_rng, rng in zip(shared_rngs, rng_gen.spawn(n_shared_rngs), strict=True)

tests/sampling/test_mcmc.py

@@ -929,12 +929,29 @@ def trace_backend(request):
         return trace
 
 
-def test_random_deterministics(trace_backend):
+@pytest.fixture(scope="function", params=["FAST_COMPILE", "NUMBA", "JAX"])
+def pytensor_mode(request):
+    return request.param
+
+
+def test_random_deterministics(trace_backend, pytensor_mode):
     with pm.Model() as m:
         x = pm.Bernoulli("x", p=0.5) * 0  # Force it to be zero
         pm.Deterministic("y", x + pm.Normal.dist())
 
-        idata1 = pm.sample(tune=0, draws=1, random_seed=1, trace=trace_backend)
-        idata2 = pm.sample(tune=0, draws=1, random_seed=1, trace=trace_backend)
-
-    assert idata1.posterior.equals(idata2.posterior)
+        if pytensor_mode == "JAX":
+            expected_warning = (
+                "At the moment, it is not possible to set the random generator's key for "
+                "JAX linked functions. This means that the draws yielded by the random "
+                "variables that are requested by 'Deterministic' will not be reproducible."
+            )
+            with pytest.warns(UserWarning, match=expected_warning):
+                with pytensor.config.change_flags(mode=pytensor_mode):
+                    idata1 = pm.sample(tune=0, draws=1, random_seed=1, trace=trace_backend)
+                    idata2 = pm.sample(tune=0, draws=1, random_seed=1, trace=trace_backend)
+            assert not idata1.posterior.equals(idata2.posterior)
+        else:
+            with pytensor.config.change_flags(mode=pytensor_mode):
+                idata1 = pm.sample(tune=0, draws=1, random_seed=1, trace=trace_backend)
+                idata2 = pm.sample(tune=0, draws=1, random_seed=1, trace=trace_backend)
+            assert idata1.posterior.equals(idata2.posterior)