Refactor _init_trace out of mcmc module

This uncouples several things:
* `_init_trace` is now independent of abstract step-methods
* `mcmc` is now unaware of `NDArray`
* code for population-sampling can now be extracted from `mcmc`

pymc/backends/__init__.py

@@ -60,7 +60,34 @@
 Saved backends can be loaded using `arviz.from_netcdf`
 
 """
+from copy import copy
+from typing import Dict, List, Optional
+
 from pymc.backends.arviz import predictions_to_inference_data, to_inference_data
+from pymc.backends.base import BaseTrace
 from pymc.backends.ndarray import NDArray, point_list_to_multitrace
 
 __all__ = ["to_inference_data", "predictions_to_inference_data"]
+
+
+def _init_trace(
+    *,
+    expected_length: int,
+    chain_number: int,
+    stats_dtypes: List[Dict[str, type]],
+    trace: Optional[BaseTrace],
+    model,
+) -> BaseTrace:
+    """Initializes a trace backend for a chain."""
+    strace: BaseTrace
+    if trace is None:
+        strace = NDArray(model=model)
+    elif isinstance(trace, BaseTrace):
+        if len(trace) > 0:
+            raise ValueError("Continuation of traces is no longer supported.")
+        strace = copy(trace)
+    else:
+        raise NotImplementedError(f"Unsupported `trace`: {trace}")
+
+    strace.setup(expected_length, chain_number, stats_dtypes)
+    return strace

pymc/sampling/mcmc.py

@@ -34,8 +34,8 @@
 
 import pymc as pm
 
+from pymc.backends import _init_trace
 from pymc.backends.base import BaseTrace, MultiTrace, _choose_chains
-from pymc.backends.ndarray import NDArray
 from pymc.blocking import DictToArrayBijection
 from pymc.exceptions import SamplingError
 from pymc.initial_point import (
@@ -960,7 +960,7 @@ def _iter_sample(
 
     strace: BaseTrace = _init_trace(
         expected_length=draws + tune,
-        step=step,
+        stats_dtypes=step.stats_dtypes,
         chain_number=chain,
         trace=trace,
         model=model,
@@ -1229,7 +1229,7 @@ def _prepare_iter_population(
     traces: List[BaseTrace] = [
         _init_trace(
             expected_length=draws + tune,
-            step=steppers[c],
+            stats_dtypes=steppers[c].stats_dtypes,
             chain_number=c,
             trace=None,
             model=model,
@@ -1306,32 +1306,6 @@ def _iter_population(
                 steppers[c].report._finalize(strace)
 
 
-def _init_trace(
-    *,
-    expected_length: int,
-    step: Step,
-    chain_number: int,
-    trace: Optional[BaseTrace],
-    model,
-) -> BaseTrace:
-    """Extracted helper function to create trace backends for each chain."""
-    strace: BaseTrace
-    if trace is None:
-        strace = NDArray(model=model)
-    elif isinstance(trace, BaseTrace):
-        if len(trace) > 0:
-            raise ValueError("Continuation of traces is no longer supported.")
-        strace = copy(trace)
-    else:
-        raise NotImplementedError(f"Unsupported `trace`: {trace}")
-
-    if step.generates_stats:
-        strace.setup(expected_length, chain_number, step.stats_dtypes)
-    else:
-        strace.setup(expected_length, chain_number)
-    return strace
-
-
 def _mp_sample(
     draws: int,
     tune: int,
@@ -1393,7 +1367,7 @@ def _mp_sample(
     traces = [
         _init_trace(
             expected_length=draws + tune,
-            step=step,
+            stats_dtypes=step.stats_dtypes,
             chain_number=chain_number,
             trace=trace,
             model=model,

pymc/tests/backends/test_base.py

@@ -14,6 +14,9 @@
 import numpy as np
 import pytest
 
+import pymc as pm
+
+from pymc.backends import _init_trace
 from pymc.backends.base import _choose_chains
 
 
@@ -31,3 +34,22 @@ def test_choose_chains(n_points, tune, expected_length, expected_n_traces):
     traces, length = _choose_chains([trace_0, trace_1, trace_2], tune=tune)
     assert length == expected_length
     assert expected_n_traces == len(traces)
+
+
+class TestInitTrace:
+    def test_init_trace_continuation_unsupported(self):
+        with pm.Model() as pmodel:
+            A = pm.Normal("A")
+            B = pm.Uniform("B")
+            strace = pm.backends.ndarray.NDArray(vars=[A, B])
+            strace.setup(10, 0)
+            strace.record({"A": 2, "B_interval__": 0.1})
+            assert len(strace) == 1
+            with pytest.raises(ValueError, match="Continuation of traces"):
+                _init_trace(
+                    expected_length=20,
+                    stats_dtypes=pm.Metropolis().stats_dtypes,
+                    chain_number=0,
+                    trace=strace,
+                    model=pmodel,
+                )

pymc/tests/sampling/test_mcmc.py

@@ -544,25 +544,6 @@ def test_constant_named(self):
             assert np.isclose(res, 0.0)
 
 
-class TestInitTrace:
-    def test_init_trace_continuation_unsupported(self):
-        with pm.Model() as pmodel:
-            A = pm.Normal("A")
-            B = pm.Uniform("B")
-            strace = pm.backends.ndarray.NDArray(vars=[A, B])
-            strace.setup(10, 0)
-            strace.record({"A": 2, "B_interval__": 0.1})
-            assert len(strace) == 1
-            with pytest.raises(ValueError, match="Continuation of traces"):
-                pm.sampling.mcmc._init_trace(
-                    expected_length=20,
-                    step=pm.Metropolis(),
-                    chain_number=0,
-                    trace=strace,
-                    model=pmodel,
-                )
-
-
 def check_exec_nuts_init(method):
     with pm.Model() as model:
         pm.Normal("a", mu=0, sigma=1, size=2)