Improve error message for missing get_moment implementations

aseyboldt · michaelosthege · commit 2b9e465b2383 · 2021-09-24T23:01:38.000+02:00
diff --git a/pymc/distributions/distribution.py b/pymc/distributions/distribution.py
@@ -351,7 +351,9 @@ def dist(
 
 @singledispatch
 def _get_moment(op, rv, size, *rv_inputs) -> TensorVariable:
-    return None
+    raise NotImplementedError(
+        f"Random variable {rv} of type {op} has no get_moment implementation."
+    )
 
 
 def get_moment(rv: TensorVariable) -> TensorVariable:
