Mark object stats as str-typed

pymc/backends/mcbackend.py

@@ -221,9 +221,13 @@ def make_runmeta_and_point_fn(
                 (-1 if s is None else s)
                 for s in (shape or [])
             ]
+            dt = np.dtype(dtype).name
+            # Object types will be pickled by the ChainRecordAdapter!
+            if dt == "object":
+                dt = "str"
             svar = mcb.Variable(
                 name=sname,
-                dtype=np.dtype(dtype).name,
+                dtype=dt,
                 shape=sshape,
                 undefined_ndim=shape is None,
             )

tests/backends/test_mcbackend.py

@@ -120,6 +120,19 @@ def test_make_runmeta_and_point_fn(simple_model):
     assert not vars["vector_interval__"].is_deterministic
     assert vars["matrix"].is_deterministic
     assert len(rmeta.sample_stats) == len(step.stats_dtypes[0])
+
+    with simple_model:
+        step = pm.NUTS()
+    rmeta, point_fn = make_runmeta_and_point_fn(
+        initial_point=simple_model.initial_point(),
+        step=step,
+        model=simple_model,
+    )
+    assert isinstance(rmeta, mcb.RunMeta)
+    svars = {s.name: s for s in rmeta.sample_stats}
+    # Unbeknownst to McBackend, object stats are pickled to str
+    assert "sampler_0__warning" in svars
+    assert svars["sampler_0__warning"].dtype == "str"
     pass