fix unpickling PointFunc bug and add regression test

Author: jessegrabowski

pymc/pytensorf.py

@@ -569,8 +569,15 @@ def __call__(self, state):
 
     def __getattr__(self, item):
         """Allow access to the original function attributes."""
-        # This is only reached if `__getattribute__` fails.
-        return getattr(self.f, item)
+        # During unpickling, attribute access will occur before self.f is set.
+        # This leads to an infinite loop, trying to call __getattr__ on self.f.
+        # To break this loop, first check if self.f is set, and raise if not.
+        try:
+            f = object.__getattribute__(self, "f")
+        except AttributeError:
+            raise AttributeError(item)
+
+        return getattr(f, item)
 
 
 class CallableTensor:

tests/step_methods/test_metropolis.py

@@ -425,3 +425,20 @@ def test_sampling_state(step_method, model_fn):
             assert equal_sampling_states(final_state1, final_state2)
             assert equal_dataclass_values(sample1, sample2)
             assert equal_dataclass_values(stat1, stat2)
+
+
+def test_binary_gibbs_with_spawn():
+    # Regression test for https://github.com/pymc-devs/pymc/issues/7857
+
+    with pm.Model():
+        x = pm.Categorical("x", logit_p=[1.0, 1.0, 1.0, 1.0])
+        idata = pm.sample(
+            step=pm.CategoricalGibbsMetropolis([x]),
+            mp_ctx="spawn",
+            compute_convergence_checks=False,
+            chains=2,
+            tune=0,
+            draws=50,
+        )
+
+    assert idata