Fix expand change_dist_size of SymbolicRandomVariables with size=None

pymc/distributions/distribution.py

@@ -400,7 +400,7 @@ def change_symbolic_rv_size(op: SymbolicRandomVariable, rv, new_size, expand) ->
 
     params = op.dist_params(rv.owner)
 
-    if expand:
+    if expand and not rv_size_is_none(size):
         new_size = tuple(new_size) + tuple(size)
 
     return op.rv_op(*params, size=new_size)

tests/distributions/test_distribution.py

@@ -23,7 +23,8 @@
 import scipy.stats as st
 
 from pytensor import shared
-from pytensor.tensor import TensorVariable
+from pytensor.tensor import NoneConst, TensorVariable
+from pytensor.tensor.random.utils import normalize_size_param
 
 import pymc as pm
 
@@ -43,7 +44,7 @@
 )
 from pymc.distributions.shape_utils import change_dist_size
 from pymc.logprob.basic import conditional_logp, logp
-from pymc.pytensorf import compile
+from pymc.pytensorf import compile, normalize_rng_param
 from pymc.testing import (
     BaseTestDistributionRandom,
     I,
@@ -210,6 +211,27 @@ def test_recreate_with_different_rng_inputs(self):
         new_next_rng, new_x = x.owner.op(*inputs)
         assert op.update(new_x.owner) == {new_rng: new_next_rng}
 
+    def test_change_dist_size_none(self):
+        class TestRV(SymbolicRandomVariable):
+            extended_signature = "[rng],[size]->[rng],(n)"
+
+            @classmethod
+            def rv_op(cls, size=None, rng=None):
+                rng = normalize_rng_param(rng)
+                size = normalize_size_param(size)
+                next_rng, draws = Normal.dist(size=size, rng=rng).owner.outputs
+                return cls(inputs=[rng, size], outputs=[next_rng, draws])(rng, size)
+
+        size = NoneConst
+        rv = TestRV.rv_op(size=size)
+        assert rv.type.shape == ()
+
+        resized_rv = change_dist_size(rv, new_size=5)
+        assert resized_rv.type.shape == (5,)
+
+        resized_rv = change_dist_size(rv, new_size=5, expand=True)
+        assert resized_rv.type.shape == (5,)
+
 
 def test_tag_future_warning_dist():
     # Test no unexpected warnings

tests/distributions/test_shape_utils.py

@@ -427,6 +427,17 @@ def test_change_rv_size():
     assert tuple(rv_newer.shape.eval()) == (2,)
 
 
+def test_change_rv_size_expand_none_size():
+    x = pt.random.normal()
+    size = x.owner.op.size_param(x.owner)
+    assert rv_size_is_none(size)
+    new_x = change_dist_size(x, new_size=(2,), expand=True)
+    new_size = new_x.owner.op.size_param(new_x.owner)
+    assert not rv_size_is_none(new_size)
+    assert new_size.data == [2]
+    assert new_x.type.shape == (2,)
+
+
 def test_change_rv_size_default_update():
     rng = pytensor.shared(np.random.default_rng(0))
     x = normal(rng=rng)