Fix DiscreteMarkovChain logp

pymc_experimental/distributions/timeseries.py

@@ -19,7 +19,7 @@
 )
 from pymc.logprob.abstract import _logprob
 from pymc.logprob.basic import logp
-from pymc.pytensorf import intX
+from pymc.pytensorf import constant_fold, intX
 from pymc.util import check_dist_not_registered
 from pytensor.graph.basic import Node
 from pytensor.tensor import TensorVariable
@@ -252,11 +252,16 @@ def discrete_mc_logp(op, values, P, steps, init_dist, state_rng, **kwargs):
     mc_logprob = logp(init_dist, value[..., :n_lags]).sum(axis=-1)
     mc_logprob += pt.log(P[tuple(indexes)]).sum(axis=-1)
 
+    # We cannot leave any RV in the logp graph, even if just for an assert
+    [init_dist_leading_dim] = constant_fold(
+        [pt.atleast_1d(init_dist).shape[0]], raise_not_constant=False
+    )
+
     return check_parameters(
         mc_logprob,
         pt.all(pt.eq(P.shape[-(n_lags + 1) :], P.shape[-1])),
         pt.all(pt.allclose(P.sum(axis=-1), 1.0)),
-        pt.eq(pt.atleast_1d(init_dist).shape[0], n_lags),
+        pt.eq(init_dist_leading_dim, n_lags),
         msg="Last (n_lags + 1) dimensions of P must be square, "
         "P must sum to 1 along the last axis, "
         "First dimension of init_dist must be n_lags",

pymc_experimental/tests/distributions/test_discrete_markov_chain.py

@@ -90,10 +90,19 @@ def test_logp_with_default_init_dist(self):
         P = pt.as_tensor_variable(np.array([[0.1, 0.5, 0.4], [0.3, 0.4, 0.3], [0.9, 0.05, 0.05]]))
         x0 = pm.Categorical.dist(p=np.ones(3) / 3)
 
-        chain = DiscreteMarkovChain.dist(P=P, init_dist=x0, steps=3)
+        value = np.array([0, 1, 2])
+        logp_expected = np.log((1 / 3) * 0.5 * 0.3)
 
-        logp = pm.logp(chain, [0, 1, 2]).eval()
-        assert logp == pytest.approx(np.log((1 / 3) * 0.5 * 0.3), rel=1e-6)
+        # Test dist directly
+        chain = DiscreteMarkovChain.dist(P=P, init_dist=x0, steps=3)
+        logp_eval = pm.logp(chain, value).eval()
+        np.testing.assert_allclose(logp_eval, logp_expected, rtol=1e-6)
+
+        # Test via Model
+        with pm.Model() as m:
+            DiscreteMarkovChain("chain", P=P, init_dist=x0, steps=3)
+        model_logp_eval = m.compile_logp()({"chain": value})
+        np.testing.assert_allclose(model_logp_eval, logp_expected, rtol=1e-6)
 
     def test_logp_with_user_defined_init_dist(self):
         P = pt.as_tensor_variable(np.array([[0.1, 0.5, 0.4], [0.3, 0.4, 0.3], [0.9, 0.05, 0.05]]))