extra checks for pixels (#4009)

Co-authored-by: Jonathan Shimwell <jon@proximafusion.com>
Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

openmc/model/model.py

@@ -34,6 +34,15 @@ class ModelModifier(Protocol):
         ...
 
 
+def _check_pixels(pixels: int | Sequence[int]) -> None:
+    if isinstance(pixels, Integral):
+        check_greater_than('pixels', pixels, 0)
+    else:
+        check_length('pixels', pixels, 2)
+        for p in pixels:
+            check_greater_than('pixels', p, 0)
+
+
 class Model:
     """Model container.
 
@@ -1051,12 +1060,7 @@ class Model:
         pixels: int | Sequence[int],
         basis: str
     ):
-        if isinstance(pixels, int):
-            check_greater_than('pixels', pixels, 0)
-        else:
-            check_length('pixels', pixels, 2)
-            for p in pixels:
-                check_greater_than('pixels', p, 0)
+        _check_pixels(pixels)
 
         x, y, _ = _BASIS_INDICES[basis]
 
@@ -1220,6 +1224,8 @@ class Model:
         """
         import openmc.lib
 
+        _check_pixels(pixels)
+
         if width is not None and (u_span is not None or v_span is not None):
             raise ValueError("width is mutually exclusive with u_span/v_span.")
 

tests/unit_tests/test_model.py

@@ -679,6 +679,8 @@ def test_model_plot_invalid_inputs():
         model.plot(pixels=(0, 100))
     with pytest.raises(ValueError):
         model.plot(pixels=(100,))
+    with pytest.raises(ValueError):
+        model.slice_data(u_span=(2, 0, 0), v_span=(0, 2, 0), pixels=-1)
 
 
 def test_model_id_map_initialization(run_in_tmpdir):