Set rng state for trace fn mapping draws to posterior samples

Co-authored-by: Ricardo Vieira <[email protected]>

pymc/backends/__init__.py

@@ -76,6 +76,7 @@
 from pymc.blocking import PointType
 from pymc.model import Model
 from pymc.step_methods.compound import BlockedStep, CompoundStep
+from pymc.util import get_random_generator
 
 HAS_MCB = False
 try:
@@ -103,11 +104,13 @@ def _init_trace(
     model: Model,
     trace_vars: list[TensorVariable] | None = None,
     initial_point: PointType | None = None,
+    rng: np.random.Generator | None = None,
 ) -> BaseTrace:
     """Initialize a trace backend for a chain."""
+    rng_ = get_random_generator(rng)
     strace: BaseTrace
     if trace is None:
-        strace = NDArray(model=model, vars=trace_vars, test_point=initial_point)
+        strace = NDArray(model=model, vars=trace_vars, test_point=initial_point, rng=rng_)
     elif isinstance(trace, BaseTrace):
         if len(trace) > 0:
             raise ValueError("Continuation of traces is no longer supported.")
@@ -129,6 +132,7 @@ def init_traces(
     model: Model,
     trace_vars: list[TensorVariable] | None = None,
     tune: int = 0,
+    rng: np.random.Generator | None = None,
 ) -> tuple[RunType | None, Sequence[IBaseTrace]]:
     """Initialize a trace recorder for each chain."""
     if isinstance(backend, ZarrTrace):
@@ -140,6 +144,7 @@ def init_traces(
             model=model,
             vars=trace_vars,
             test_point=initial_point,
+            rng=rng,
         )
         return None, backend.straces
     if HAS_MCB and isinstance(backend, Backend):
@@ -149,6 +154,7 @@ def init_traces(
             initial_point=initial_point,
             step=step,
             model=model,
+            rng=rng,
         )
 
     assert backend is None or isinstance(backend, BaseTrace)
@@ -161,7 +167,8 @@ def init_traces(
             model=model,
             trace_vars=trace_vars,
             initial_point=initial_point,
+            rng=rng_,
         )
-        for chain_number in range(chains)
+        for chain_number, rng_ in enumerate(get_random_generator(rng).spawn(chains))
     ]
     return None, traces

pymc/backends/base.py

@@ -34,7 +34,7 @@
 
 from pymc.backends.report import SamplerReport
 from pymc.model import modelcontext
-from pymc.pytensorf import compile
+from pymc.pytensorf import compile, set_function_rngs
 from pymc.util import get_var_name
 
 logger = logging.getLogger(__name__)
@@ -159,6 +159,7 @@ def __init__(
         fn=None,
         var_shapes=None,
         var_dtypes=None,
+        rng=None,
     ):
         model = modelcontext(model)
 
@@ -177,6 +178,8 @@ def __init__(
                 on_unused_input="ignore",
             )
             fn.trust_input = True
+        if rng is not None:
+            fn = set_function_rngs(fn=fn, rng=rng)
 
         # Get variable shapes. Most backends will need this
         # information.

pymc/backends/mcbackend.py

@@ -29,7 +29,7 @@
 
 from pymc.backends.base import IBaseTrace
 from pymc.model import Model
-from pymc.pytensorf import PointFunc
+from pymc.pytensorf import PointFunc, set_function_rngs
 from pymc.step_methods.compound import (
     BlockedStep,
     CompoundStep,
@@ -38,6 +38,7 @@
     flat_statname,
     flatten_steps,
 )
+from pymc.util import get_random_generator
 
 _log = logging.getLogger(__name__)
 
@@ -96,7 +97,11 @@ class ChainRecordAdapter(IBaseTrace):
     """Wraps an McBackend ``Chain`` as an ``IBaseTrace``."""
 
     def __init__(
-        self, chain: mcb.Chain, point_fn: PointFunc, stats_bijection: StatsBijection
+        self,
+        chain: mcb.Chain,
+        point_fn: PointFunc,
+        stats_bijection: StatsBijection,
+        rng: np.random.Generator | None = None,
     ) -> None:
         # Assign attributes required by IBaseTrace
         self.chain = chain.cmeta.chain_number
@@ -107,8 +112,11 @@ def __init__(
             for sstats in stats_bijection._stat_groups
         ]
 
+        self._rng = rng
         self._chain = chain
         self._point_fn = point_fn
+        if rng is not None:
+            self._point_fn = set_function_rngs(self._point_fn, rng)
         self._statsbj = stats_bijection
         super().__init__()
 
@@ -257,6 +265,7 @@ def init_chain_adapters(
     initial_point: Mapping[str, np.ndarray],
     step: CompoundStep | BlockedStep,
     model: Model,
+    rng: np.random.Generator | None,
 ) -> tuple[mcb.Run, list[ChainRecordAdapter]]:
     """Create an McBackend metadata description for the MCMC run.
 
@@ -286,7 +295,8 @@ def init_chain_adapters(
             chain=run.init_chain(chain_number=chain_number),
             point_fn=point_fn,
             stats_bijection=statsbj,
+            rng=rng_,
         )
-        for chain_number in range(chains)
+        for chain_number, rng_ in enumerate(get_random_generator(rng).spawn(chains))
     ]
     return run, adapters

pymc/backends/zarr.py

@@ -35,13 +35,20 @@
 from pymc.backends.base import BaseTrace
 from pymc.blocking import StatDtype, StatShape
 from pymc.model.core import Model, modelcontext
+from pymc.pytensorf import set_function_rngs
 from pymc.step_methods.compound import (
     BlockedStep,
     CompoundStep,
     StatsBijection,
     get_stats_dtypes_shapes_from_steps,
 )
-from pymc.util import UNSET, _UnsetType, get_default_varnames, is_transformed_name
+from pymc.util import (
+    UNSET,
+    _UnsetType,
+    get_default_varnames,
+    get_random_generator,
+    is_transformed_name,
+)
 
 try:
     from zarr.storage import BaseStore, default_compressor
@@ -398,6 +405,7 @@ def init_trace(
         model: Model | None = None,
         vars: Sequence[TensorVariable] | None = None,
         test_point: dict[str, np.ndarray] | None = None,
+        rng: np.random.Generator | None = None,
     ):
         """Initialize the trace groups and arrays.
 
@@ -437,6 +445,12 @@ def init_trace(
             This is not used and is a product of the inheritance of :class:`ZarrChain`
             from :class:`~.BaseTrace`, which uses it to determine the shape and dtype
             of `vars`.
+        rng : numpy.random.Generator | None
+            A random generator to use to seed the shared random generators that are
+            present in the pytensor function that maps samples drawn by step methods
+            onto samples in the posterior trace. Note that this only does anything
+            if there are deterministic variables that are generated by raw pytensor
+            random variables.
         """
         if self._is_base_setup:
             raise RuntimeError("The ZarrTrace has already been initialized")  # pragma: no cover
@@ -534,9 +548,9 @@ def init_trace(
                 test_point=test_point,
                 stats_bijection=StatsBijection(step.stats_dtypes),
                 draws_per_chunk=self.draws_per_chunk,
-                fn=self.fn,
+                fn=set_function_rngs(self.fn, rng_),
             )
-            for _ in range(chains)
+            for rng_ in get_random_generator(rng).spawn(chains)
         ]
         for chain, strace in enumerate(self.straces):
             strace.setup(draws=tune + draws, chain=chain, sampler_vars=None)

pymc/pytensorf.py

@@ -14,14 +14,15 @@
 import warnings
 
 from collections.abc import Callable, Generator, Iterable, Sequence
-from typing import cast
+from typing import cast, overload
 
 import numpy as np
 import pandas as pd
 import pytensor
 import pytensor.tensor as pt
 import scipy.sparse as sps
 
+from pytensor import shared
 from pytensor.compile import Function, Mode, get_mode
 from pytensor.compile.builders import OpFromGraph
 from pytensor.gradient import grad
@@ -42,7 +43,7 @@
 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.type import RandomGeneratorType, RandomType
 from pytensor.tensor.random.var import RandomGeneratorSharedVariable
 from pytensor.tensor.rewriting.basic import topo_unconditional_constant_folding
 from pytensor.tensor.rewriting.shape import ShapeFeature
@@ -51,7 +52,7 @@
 from pytensor.tensor.variable import TensorVariable
 
 from pymc.exceptions import NotConstantValueError
-from pymc.util import makeiter
+from pymc.util import RandomGeneratorState, makeiter, random_generator_from_state
 from pymc.vartypes import continuous_types, isgenerator, typefilter
 
 PotentialShapeType = int | np.ndarray | Sequence[int | Variable] | TensorVariable
@@ -1163,3 +1164,53 @@ def normalize_rng_param(rng: None | Variable) -> Variable:
             "The type of rng should be an instance of either RandomGeneratorType or RandomStateType"
         )
     return rng
+
+
+@overload
+def set_function_rngs(
+    fn: PointFunc, rng: np.random.Generator | RandomGeneratorState
+) -> PointFunc: ...
+
+
+@overload
+def set_function_rngs(
+    fn: Function, rng: np.random.Generator | RandomGeneratorState
+) -> Function: ...
+
+
+def set_function_rngs(fn: Function, rng: np.random.Generator | RandomGeneratorState) -> Function:
+    """Copy a compiled pytensor function and replace the random Generators with spawns.
+
+    Parameters
+    ----------
+    fn : pytensor.compile.function.types.Function | pymc.util.PointFunc
+        The compiled function
+    rng : numpy.random.Generator | RandomGeneratorState
+        The random generator or its state
+
+    Returns
+    -------
+    fn_out : pytensor.compile.function.types.Function | pymc.pytensorf.PointFunc
+        A copy of the input function with the shared random generator states set to
+        spawns of the supplied ``rng``. If the function has no shared random generators
+        in it, the input ``fn`` is returned without any changes.
+        If ``fn`` is a :clas:`~pymc.pytensorf.PointFunc` instance, and the inner
+        pytensor function has random variables, then the inner pytensor function is
+        copied, setting new random generators, and a new ``PointFunc`` instance is
+        returned.
+    """
+    # Copy the function and replace any shared RNGs
+    # This is needed so that it can work correctly with multiple traces
+    # This will be costly if set_rng is called too often!
+    rng_gen = rng if isinstance(rng, np.random.Generator) else random_generator_from_state(rng)
+    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)
+    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)
+    }
+    if isinstance(fn, PointFunc):
+        return PointFunc(fn.f.copy(swap=swap)) if n_shared_rngs > 0 else fn
+    else:
+        return fn.copy(swap=swap) if n_shared_rngs > 0 else fn

pymc/sampling/mcmc.py

@@ -866,6 +866,7 @@ def joined_blas_limiter():
         initial_point=initial_points[0],
         model=model,
         tune=tune,
+        rng=rngs[0].spawn(1)[0],
     )
 
     sample_args = {

tests/sampling/test_mcmc.py

@@ -909,3 +909,32 @@ def test_sample(self, seeded_test):
         np.testing.assert_allclose(
             x_pred, pp_trace1.posterior_predictive["obs"].mean(("chain", "draw")), atol=1e-1
         )
+
+
+@pytest.fixture(scope="function", params=[None, "mcbackend", "zarr"])
+def trace_backend(request):
+    if request.param is None:
+        return None
+    elif request.param == "mcbackend":
+        try:
+            import mcbackend as mcb
+        except ImportError:
+            pytest.skip("Requires McBackend to be installed.")
+        return mcb.NumPyBackend()
+    elif request.param == "zarr":
+        try:
+            trace = pm.backends.zarr.ZarrTrace()
+        except RuntimeError:
+            pytest.skip("Requires zarr to be installed")
+        return trace
+
+
+def test_random_deterministics(trace_backend):
+    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)