Numba fallback cython missing dtype

pytensor/link/numba/dispatch/scalar.py

@@ -69,15 +69,19 @@ def numba_funcify_ScalarOp(op, node, **kwargs):
 
         cython_func = getattr(scipy.special.cython_special, scalar_func_name, None)
         if cython_func is not None:
-            scalar_func_numba = wrap_cython_function(
-                cython_func, output_dtype, input_dtypes
-            )
-            has_pyx_skip_dispatch = scalar_func_numba.has_pyx_skip_dispatch
-            input_inner_dtypes = scalar_func_numba.numpy_arg_dtypes()
-            output_inner_dtype = scalar_func_numba.numpy_output_dtype()
+            try:
+                scalar_func_numba = wrap_cython_function(
+                    cython_func, output_dtype, input_dtypes
+                )
+            except NotImplementedError:
+                pass
+            else:
+                has_pyx_skip_dispatch = scalar_func_numba.has_pyx_skip_dispatch
+                input_inner_dtypes = scalar_func_numba.numpy_arg_dtypes()
+                output_inner_dtype = scalar_func_numba.numpy_output_dtype()
 
     if scalar_func_numba is None:
-        scalar_func_numba = generate_fallback_impl(op, node, **kwargs)
+        return generate_fallback_impl(op, node, **kwargs), None
 
     scalar_op_fn_name = get_name_for_object(scalar_func_numba)
     prefix = "x" if scalar_func_name != "x" else "y"

tests/link/numba/test_scalar.py

 import numpy as np
 import pytest
+import scipy
 
 import pytensor.scalar as ps
 import pytensor.scalar.basic as psb
 import pytensor.scalar.math as psm
 import pytensor.tensor as pt
 from pytensor import config, function
+from pytensor.graph import Apply
+from pytensor.scalar import UnaryScalarOp
 from pytensor.scalar.basic import Composite
 from pytensor.tensor import tensor
 from pytensor.tensor.elemwise import Elemwise
@@ -184,3 +187,32 @@ def test_Softplus(dtype):
             strict=True,
             err_msg=f"Failed for value {value}",
         )
+
+
+def test_cython_obj_mode_fallback():
+    """Test that unsupported cython signatures fallback to obj-mode"""
+
+    # Create a ScalarOp with a non-standard dtype
+    class IntegerGamma(UnaryScalarOp):
+        # We'll try to check for scipy cython impl
+        nfunc_spec = ("scipy.special.gamma", 1, 1)
+
+        def make_node(self, x):
+            x = psb.as_scalar(x)
+            assert x.dtype == "int64"
+            out = x.type()
+            return Apply(self, [x], [out])
+
+        def impl(self, x):
+            return scipy.special.gamma(x).astype("int64")
+
+    x = pt.scalar("x", dtype="int64")
+    g = Elemwise(IntegerGamma())(x)
+    assert g.type.dtype == "int64"
+
+    with pytest.warns(UserWarning, match="Numba will use object mode"):
+        compare_numba_and_py(
+            [x],
+            [g],
+            [np.array(5, dtype="int64")],
+        )