New sample_posterior_predictive API

Author: ricardoV94

pymc/exceptions.py

@@ -13,6 +13,7 @@
 #   limitations under the License.
 
 __all__ = [
+    "ImplicitFreezeWarning",
     "ImputationWarning",
     "IncorrectArgumentsError",
     "SamplingError",
@@ -42,6 +43,17 @@ class ImputationWarning(UserWarning):
     pass
 
 
+class ImplicitFreezeWarning(UserWarning):
+    """Warning that trace values are being reused instead of resampled.
+
+    Emitted by ``sample_posterior_predictive`` when a trace basic RV has a
+    volatile upstream (changed Data/coords or a resampled ancestor) but is not
+    listed in ``sample_vars``.
+    """
+
+    pass
+
+
 class ShapeWarning(UserWarning):
     """Something that could lead to shape problems down the line."""