Performance: lazy-load material data on first access instead of at import

src/NeSST/constants.py

@@ -34,4 +34,19 @@
 # Materials
 default_mat_list = ["H", "D", "T", "C12", "Be9"]
 available_materials = []
-mat_dict = {}
+
+
+class LazyMaterialDict(dict):
+    """A dict that lazily loads default material data on first access."""
+
+    def __missing__(self, label):
+        from NeSST.core import initialise_material_data  # lazy import avoids circular dependency
+
+        # initialise_material_data populates self[label] for valid labels
+        initialise_material_data(label)
+        if label not in self:
+            raise KeyError(label)
+        return self[label]
+
+
+mat_dict = LazyMaterialDict()

src/NeSST/core.py

@@ -53,9 +53,6 @@ def initialise_material_data(label):
     available_materials.append(label)
 
 
-for mat in default_mat_list:
-    initialise_material_data(mat)
-
 ##########################################
 # Primary spectral shapes & reactivities #
 ##########################################

src/NeSST/spectral_model.py

@@ -29,6 +29,8 @@ def __init__(self, label, json):
         self.label = label
 
         self.json = json
+        print(f">> NeSST: First usage of material {label}")
+        print(f">> NeSST: Loading cross section data for {label}, with config {json}...")
         ENDF_data = retrieve_ENDF_data(self.json)
 
         self.A = ENDF_data["A"]

tests/test_core.py

@@ -1,6 +1,7 @@
 import NeSST as nst
 import numpy as np
 import pytest
+from NeSST.constants import LazyMaterialDict
 
 
 def test_DTprimspecmoments_mean():
@@ -75,3 +76,25 @@ def test_DTprimspecmoments_variance_relative_size():
     assert np.isclose((100 / DTmean) * DTstddev, 1, atol=0.3)
     # Check variance is standard deviation squared
     assert np.isclose(DTvar, DTstddev**2)
+
+
+def test_mat_dict_is_lazy():
+    # mat_dict should be a LazyMaterialDict (not loaded eagerly at import)
+    assert isinstance(nst.mat_dict, LazyMaterialDict)
+
+
+def test_mat_dict_lazy_loads_on_access():
+    # mat_dict should lazily load material data on first access and then cache it
+    # Be9 is accessed last among default materials, so test with it
+    label = "Be9"
+    # Ensure the label is accessible (loads lazily if not already cached)
+    mat = nst.mat_dict[label]
+    assert label in nst.mat_dict
+    # Subsequent access returns the same cached object
+    assert mat is nst.mat_dict[label]
+
+
+def test_mat_dict_raises_for_unknown_label():
+    # Accessing an unknown material label should raise KeyError
+    with pytest.raises(KeyError):
+        nst.mat_dict["unknown_material_xyz"]