Fix set_parameters on device models

src/examodels/model.py

@@ -119,8 +119,11 @@ def parameters(self, block):
 
     def set_parameters(self, block, values):
         """Change a parameter block's values in place; the model is reused as is."""
+        # Place the values where the model's arrays live, like every other
+        # setter: writing a host array into device memory falls into a scalar
+        # path the backend disallows.
         _b.guard(_b.EM.set_value_b, self._jl, block._jl,
-                 np.asarray(values, dtype=np.float64).ravel())
+                 _b.upload(self._jl, np.asarray(values, dtype=np.float64).ravel()))
         return self
 
     def solve(self, solver=None, **options):

tests/test_device_parameters.py

@@ -0,0 +1,29 @@
+"""`set_parameters` on a device model — needs hardware.
+
+Every other setter places its values where the model's arrays live
+(`_b.upload`); `set_parameters` passed the host array straight through, which
+on a device model drops into the scalar path GPUArrays forbids and raised
+`ModelError: Scalar indexing is disallowed`. The scalar-indexing guard only
+exists on a real device array, so this cannot be pinned by the stubbed-device
+tests in `test_device_interop.py`.
+"""
+import numpy as np
+import pytest
+
+import examodels as exa
+
+pytestmark = pytest.mark.skipif(
+    not __import__("shutil").which("nvidia-smi"), reason="no GPU on this machine")
+
+
+def test_set_parameters_takes_effect_on_a_device_model():
+    core = exa.Core(backend="cuda")
+    x = core.add_var(4, start=0.0)
+    p = core.add_par([1.0, 2.0, 3.0, 4.0])
+    core.add_obj(lambda i: (x[i] - p[i]) ** 2, over=range(4))
+    m = exa.Model(core)
+
+    assert m.objective(np.zeros(4)) == pytest.approx(1.0 + 4.0 + 9.0 + 16.0)
+
+    m.set_parameters(p, [5.0, 6.0, 7.0, 8.0])
+    assert m.objective(np.zeros(4)) == pytest.approx(25.0 + 36.0 + 49.0 + 64.0)