Add support for negative axis in `specify_broadcastable` (#710)

pytensor/tensor/shape.py

@@ -4,6 +4,7 @@ from textwrap import dedent
 from typing import cast
 
 import numpy as np
+from numpy.core.numeric import normalize_axis_tuple  # type: ignore
 
 import pytensor
 from pytensor.gradient import DisconnectedType
@@ -994,9 +995,7 @@ def specify_broadcastable(x, *axes):
     if not axes:
         return x
 
-    if max(axes) >= x.type.ndim:
-        raise ValueError("Trying to specify broadcastable of non-existent dimension")
-
+    axes = normalize_axis_tuple(axes, x.type.ndim)
     shape_info = [1 if i in axes else s for i, s in enumerate(x.type.shape)]
     return specify_shape(x, shape_info)
 

tests/tensor/test_shape.py

@@ -562,16 +562,22 @@ class TestSpecifyBroadcastable:
         x = matrix()
         assert specify_broadcastable(x, 0).type.shape == (1, None)
         assert specify_broadcastable(x, 1).type.shape == (None, 1)
+        assert specify_broadcastable(x, -1).type.shape == (None, 1)
         assert specify_broadcastable(x, 0, 1).type.shape == (1, 1)
 
         x = row()
         assert specify_broadcastable(x, 0) is x
         assert specify_broadcastable(x, 1) is not x
+        assert specify_broadcastable(x, -2) is x
 
     def test_validation(self):
         x = matrix()
-        with pytest.raises(ValueError, match="^Trying to specify broadcastable of*"):
-            specify_broadcastable(x, 2)
+        axis = 2
+        with pytest.raises(
+            ValueError,
+            match=f"axis {axis} is out of bounds for array of dimension {axis}",
+        ):
+            specify_broadcastable(x, axis)
 
 
 class TestRopLop(RopLopChecker):