Add fallback to prior when moment is not implemented

pymc/initial_point.py

@@ -12,6 +12,7 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 import functools
+import warnings
 
 from typing import Callable, Dict, List, Optional, Sequence, Set, Union
 
@@ -269,7 +270,19 @@ def make_initial_point_expression(
 
         if isinstance(strategy, str):
             if strategy == "moment":
-                value = get_moment(variable)
+                try:
+                    value = get_moment(variable)
+                except NotImplementedError:
+                    warnings.warn(
+                        f"Moment not defined for variable {variable} of type "
+                        f"{variable.owner.op.__class__.__name__}, defaulting to "
+                        f"a draw from the prior. This can lead to difficulties "
+                        f"during tuning. You can manually define an initval or "
+                        f"implement a get_moment dispatched function for this "
+                        f"distribution.",
+                        UserWarning,
+                    )
+                    value = variable
             elif strategy == "prior":
                 value = variable
             else:

pymc/tests/test_initial_point.py

@@ -17,6 +17,8 @@
 import numpy as np
 import pytest
 
+from aesara.tensor.random.op import RandomVariable
+
 import pymc as pm
 
 from pymc.distributions.distribution import get_moment
@@ -255,6 +257,30 @@ def test_moment_from_dims(self, rv_cls):
             assert tuple(get_moment(rv).shape.eval()) == (4, 3)
         pass
 
+    def test_moment_not_implemented_fallback(self):
+        class MyNormalRV(RandomVariable):
+            name = "my_normal"
+            ndim_supp = 0
+            ndims_params = [0, 0]
+            dtype = "floatX"
+
+            @classmethod
+            def rng_fn(cls, rng, mu, sigma, size):
+                return np.pi
+
+        class MyNormalDistribution(pm.Normal):
+            rv_op = MyNormalRV()
+
+        with pm.Model() as m:
+            x = MyNormalDistribution("x", 0, 1, initval="moment")
+
+        with pytest.warns(
+            UserWarning, match="Moment not defined for variable x of type MyNormalRV"
+        ):
+            res = m.recompute_initial_point()
+
+        assert np.isclose(res["x"], np.pi)
+
 
 def test_pickling_issue_5090():
     with pm.Model() as model: