Fix bug in shape inference of AdvancedSubtensor with slices

pytensor/tensor/subtensor.py

@@ -59,6 +59,7 @@ from pytensor.tensor.type import (
     zscalar,
 )
 from pytensor.tensor.type_other import (
+    MakeSlice,
     NoneConst,
     NoneTypeT,
     SliceConstant,
@@ -527,11 +528,20 @@ def basic_shape(shape, indices):
         if isinstance(idx, slice):
             res_shape += (slice_len(idx, n),)
         elif isinstance(getattr(idx, "type", None), SliceType):
-            if idx.owner:
-                idx_inputs = idx.owner.inputs
+            if idx.owner is None:
+                if not isinstance(idx, Constant):
+                    # This is an input slice, we can't reason symbolically on it.
+                    # We don't even know if we will get None entries or integers
+                    res_shape += (None,)
+                    continue
+                else:
+                    sl: slice = idx.data
+                    slice_inputs = (sl.start, sl.stop, sl.step)
+            elif isinstance(idx.owner.op, MakeSlice):
+                slice_inputs = idx.owner.inputs
             else:
-                idx_inputs = (None,)
-            res_shape += (slice_len(slice(*idx_inputs), n),)
+                raise ValueError(f"Unexpected Slice producing Op {idx.owner.op}")
+            res_shape += (slice_len(slice(*slice_inputs), n),)
         elif idx is None:
             res_shape += (ps.ScalarConstant(ps.int64, 1),)
         elif isinstance(getattr(idx, "type", None), NoneTypeT):
@@ -2728,6 +2738,11 @@ class AdvancedSubtensor(Op):
         res_shape = list(
             indexed_result_shape(ishapes[0], index_shapes, indices_are_shapes=True)
         )
+        for i, res_dim_length in enumerate(res_shape):
+            if res_dim_length is None:
+                # This can happen when we have a Slice provided by the user (not a constant nor the result of MakeSlice)
+                # We must compute the Op to find its shape
+                res_shape[i] = Shape_i(i)(node.out)
 
         adv_indices = [idx for idx in indices if not is_basic_idx(idx)]
         bool_indices = [idx for idx in adv_indices if is_bool_index(idx)]

tests/tensor/test_subtensor.py

@@ -15,6 +15,7 @@ from pytensor.compile.io import In
 from pytensor.compile.mode import Mode
 from pytensor.configdefaults import config
 from pytensor.gradient import grad
+from pytensor.graph import Constant
 from pytensor.graph.op import get_test_value
 from pytensor.graph.rewriting.utils import is_same_graph
 from pytensor.printing import pprint
@@ -37,6 +38,7 @@ from pytensor.tensor.subtensor import (
     advanced_inc_subtensor1,
     advanced_set_subtensor,
     advanced_set_subtensor1,
+    advanced_subtensor,
     advanced_subtensor1,
     as_index_literal,
     basic_shape,
@@ -2145,7 +2147,17 @@ class TestAdvancedSubtensor:
         slc = slicetype()
         f = pytensor.function([slc], var[slc], mode=self.mode)
         s = slice(1, 3)
-        f(s)
+        assert f(s).shape == (2, 3)
+
+        f_shape0 = pytensor.function([slc], var[slc].shape[0], mode=self.mode)
+        assert f_shape0(s) == 2
+
+        f_shape1 = pytensor.function([slc], var[slc].shape[1], mode=self.mode)
+        assert not any(
+            isinstance(node.op, AdvancedSubtensor)
+            for node in f_shape1.maker.fgraph.toposort()
+        )
+        assert f_shape1(s) == 3
 
     def test_adv_grouped(self):
         # Reported in https://github.com/Theano/Theano/issues/6152
@@ -2611,6 +2623,14 @@ class TestInferShape(utt.InferShapeTester):
             AdvancedSubtensor,
         )
 
+    def test_advanced_subtensor_constant_slice(self):
+        x = dmatrix("x")
+        constant_slice = pytensor.as_symbolic(slice(1, None, None))
+        assert isinstance(constant_slice, Constant)
+        adv_indices = ptb.constant(np.zeros((2, 3)), dtype="int")
+        y = advanced_subtensor(x, constant_slice, adv_indices)
+        assert tuple(y.shape.eval({x: np.zeros((10, 10))})) == (9, 2, 3)
+
 
 @config.change_flags(compute_test_value="raise")
 def test_basic_shape():