enh: Improve static shape of subtensor

pytensor/tensor/subtensor.py

@@ -221,6 +221,16 @@ def get_canonical_form_slice(
     step, is_step_constant = analyze(theslice.step)
     length, is_length_constant = analyze(length)
 
+    if (
+        is_start_constant
+        and is_stop_constant
+        and is_step_constant
+        and is_length_constant
+    ):
+        _start, _stop, _step = slice(start, stop, step).indices(length)
+        if _start <= _stop and _step >= 1:
+            return slice(_start, _stop, _step), 1
+
     if step is None:
         step = 1
         is_step_constant = True
@@ -722,32 +732,51 @@ class Subtensor(COp):
                     f"Incompatible types for Subtensor template. Expected {input.type}, got {expected_type}."
                 )
 
-        # infer the broadcasting pattern
-        padded = get_constant_idx(
-            self.idx_list, (None,) + inputs, allow_partial=True
-        ) + [slice(None, None, None)] * (x.type.ndim - len(idx_list))
+        padded = [
+            *get_idx_list((None,) + inputs, self.idx_list),
+            *[slice(None, None, None)] * (x.type.ndim - len(idx_list)),
+        ]
 
         out_shape = []
-        for i, (p, s) in enumerate(zip(padded, x.type.shape)):
-            if isinstance(p, slice):
-                if s == 1:
-                    start = p.start
+
+        def extract_const(value):
+            if value is None:
+                return value, True
             try:
-                        start = get_underlying_scalar_constant_value(start)
+                value = get_underlying_scalar_constant_value(value)
+                return value, True
             except NotScalarConstantError:
-                        pass
-                    if start is None or start == 0:
-                        start = p.start
-                        if start is None:
-                            start = 0
-                        if p.stop is None or (
-                            isinstance(p.stop, (int, np.integer, np.ndarray))
-                            and p.stop > start
-                        ):
-                            out_shape.append(1)
+                return value, False
+
+        for the_slice, length in zip(padded, x.type.shape):
+            if not isinstance(the_slice, slice):
                 continue
 
+            if length is None:
                 out_shape.append(None)
+                continue
+
+            start = the_slice.start
+            stop = the_slice.stop
+            step = the_slice.step
+
+            is_slice_const = True
+
+            start, is_const = extract_const(start)
+            is_slice_const = is_slice_const and is_const
+
+            stop, is_const = extract_const(stop)
+            is_slice_const = is_slice_const and is_const
+
+            step, is_const = extract_const(step)
+            is_slice_const = is_slice_const and is_const
+
+            if not is_slice_const:
+                out_shape.append(None)
+                continue
+
+            slice_length = len(range(*slice(start, stop, step).indices(length)))
+            out_shape.append(slice_length)
 
         return Apply(
             self,

tests/tensor/test_subtensor.py

@@ -2693,3 +2693,18 @@ def test_index_vars_to_types():
     assert isinstance(x.type, scal.ScalarType)
     res = index_vars_to_types(x)
     assert res == x.type
+
+
+@pytest.mark.parametrize(
+    "x_shape, indices, expected",
+    [
+        [(None,), (slice(None, None),), (None,)],
+        [(13,), (slice(None, 100),), (13,)],
+        [(13,), (slice(-1, None),), (1,)],
+        [(7, 13), (slice(None, None, 2), slice(-1, 1, -1)), (4, 11)],
+    ],
+)
+def test_static_shapes(x_shape, indices, expected):
+    x = at.tensor(dtype="float64", shape=x_shape)
+    y = x[indices]
+    assert y.type.shape == expected