Prevent Model access in random function of CustomDist

pymc/distributions/distribution.py

@@ -46,6 +46,7 @@
     find_size,
     shape_from_dims,
 )
+from pymc.exceptions import BlockModelAccessError
 from pymc.logprob.abstract import (
     MeasurableVariable,
     _get_measurable_outputs,
@@ -54,6 +55,7 @@
     _logprob,
 )
 from pymc.logprob.rewriting import logprob_rewrites_db
+from pymc.model import BlockModelAccess
 from pymc.printing import str_for_dist
 from pymc.pytensorf import collect_default_updates, convert_observed_data
 from pymc.util import UNSET, _add_future_warning_tag
@@ -662,7 +664,10 @@ def rv_op(
         size = normalize_size_param(size)
         dummy_size_param = size.type()
         dummy_dist_params = [dist_param.type() for dist_param in dist_params]
-        dummy_rv = random(*dummy_dist_params, dummy_size_param)
+        with BlockModelAccess(
+            error_msg_on_access="Model variables cannot be created in the random function. Use the `.dist` API"
+        ):
+            dummy_rv = random(*dummy_dist_params, dummy_size_param)
         dummy_params = [dummy_size_param] + dummy_dist_params
         dummy_updates_dict = collect_default_updates(dummy_params, (dummy_rv,))
 
@@ -1050,7 +1055,12 @@ def is_symbolic_random(self, random, dist_params):
         # Try calling random with symbolic inputs
         try:
             size = normalize_size_param(None)
-            out = random(*dist_params, size)
+            with BlockModelAccess(
+                error_msg_on_access="Model variables cannot be created in the random function. Use the `.dist` API"
+            ):
+                out = random(*dist_params, size)
+        except BlockModelAccessError:
+            raise
         except Exception:
             # If it fails we assume it was not
             return False

pymc/exceptions.py

@@ -82,3 +82,7 @@ class TruncationError(RuntimeError):
 
 class NotConstantValueError(ValueError):
     pass
+
+
+class BlockModelAccessError(RuntimeError):
+    pass

pymc/model.py

@@ -53,7 +53,13 @@
 from pymc.data import GenTensorVariable, Minibatch
 from pymc.distributions.logprob import _joint_logp
 from pymc.distributions.transforms import _default_transform
-from pymc.exceptions import ImputationWarning, SamplingError, ShapeError, ShapeWarning
+from pymc.exceptions import (
+    BlockModelAccessError,
+    ImputationWarning,
+    SamplingError,
+    ShapeError,
+    ShapeWarning,
+)
 from pymc.initial_point import make_initial_point_fn
 from pymc.pytensorf import (
     PointFunc,
@@ -195,6 +201,8 @@ def get_context(cls, error_if_none=True) -> Optional[T]:
             if error_if_none:
                 raise TypeError(f"No {cls} on context stack")
             return None
+        if isinstance(candidate, BlockModelAccess):
+            raise BlockModelAccessError(candidate.error_msg_on_access)
         return candidate
 
     def get_contexts(cls) -> List[T]:
@@ -1798,6 +1806,13 @@ def point_logps(self, point=None, round_vals=2):
 Model._context_class = Model
 
 
+class BlockModelAccess(Model):
+    """This class can be used to prevent user access to Model contexts"""
+
+    def __init__(self, *args, error_msg_on_access="Model access is blocked", **kwargs):
+        self.error_msg_on_access = error_msg_on_access
+
+
 def set_data(new_data, model=None, *, coords=None):
     """Sets the value of one or more data container variables.  Note that the shape is also
     dynamic, it is updated when the value is changed.  See the examples below for two common

pymc/tests/distributions/test_distribution.py

@@ -45,6 +45,7 @@
 )
 from pymc.distributions.shape_utils import change_dist_size, rv_size_is_none, to_tuple
 from pymc.distributions.transforms import log
+from pymc.exceptions import BlockModelAccessError
 from pymc.logprob.abstract import get_measurable_outputs, logcdf
 from pymc.model import Model
 from pymc.sampling import draw, sample
@@ -479,6 +480,17 @@ def custom_random(mu, sigma, size):
         assert isinstance(new_lognormal.owner.op, CustomSymbolicDistRV)
         assert tuple(new_lognormal.shape.eval()) == (2, 5, 10)
 
+    def test_error_model_access(self):
+        def random(size):
+            return pm.Flat("Flat", size=size)
+
+        with pm.Model() as m:
+            with pytest.raises(
+                BlockModelAccessError,
+                match="Model variables cannot be created in the random function",
+            ):
+                CustomDist("custom_dist", random=random)
+
 
 class TestSymbolicRandomVarible:
     def test_inline(self):