fix(rdf): avoid mutating selection defaults

dpdata/md/rdf.py

@@ -3,7 +3,7 @@
 import numpy as np
 
 
-def rdf(sys, sel_type=[None, None], max_r=5, nbins=100):
+def rdf(sys, sel_type=None, max_r=5, nbins=100):
     """Compute the rdf of a system.
 
     Parameters
@@ -40,7 +40,12 @@ def rdf(sys, sel_type=[None, None], max_r=5, nbins=100):
     )
 
 
-def compute_rdf(box, posis, atype, sel_type=[None, None], max_r=5, nbins=100):
+def compute_rdf(box, posis, atype, sel_type=None, max_r=5, nbins=100):
+    """Compute an RDF without mutating the caller's selection list."""
+    if sel_type is None:
+        sel_type = [None, None]
+    else:
+        sel_type = list(sel_type)
     nframes = box.shape[0]
     xx = None
     all_rdf = []
@@ -58,7 +63,13 @@ def compute_rdf(box, posis, atype, sel_type=[None, None], max_r=5, nbins=100):
     return xx, all_rdf, all_cod
 
 
-def _compute_rdf_1frame(box, posis, atype, sel_type=[None, None], max_r=5, nbins=100):
+def _compute_rdf_1frame(box, posis, atype, sel_type=None, max_r=5, nbins=100):
+    if sel_type is None:
+        sel_type = [None, None]
+    else:
+        # Normalise into a fresh list because the two ``None`` replacements
+        # below must not leak into a caller-owned list or a function default.
+        sel_type = list(sel_type)
     all_types = list(set(list(np.sort(atype, kind="stable"))))
     if sel_type[0] is None:
         sel_type[0] = all_types

tests/test_rdf.py

@@ -0,0 +1,34 @@
+from __future__ import annotations
+
+import unittest
+
+import numpy as np
+
+from dpdata.md.rdf import compute_rdf
+
+try:
+    import ase  # noqa: F401
+except ModuleNotFoundError:
+    skip_ase = True
+else:
+    skip_ase = False
+
+
+@unittest.skipIf(skip_ase, "RDF calculation requires ASE")
+class TestRDFSelectionOwnership(unittest.TestCase):
+    def test_default_selection_is_recomputed_per_system(self):
+        box = np.eye(3).reshape(1, 3, 3) * 10
+        positions = np.array([[[0.0, 0.0, 0.0], [1.0, 0.0, 0.0]]])
+        rdf = compute_rdf
+
+        rdf(box, positions, np.array([0, 1]))
+        self.assertEqual(rdf.__defaults__[0], None)
+        _, values, _ = rdf(box, positions, np.array([2, 2]))
+        self.assertTrue(np.isfinite(values).all())
+
+    def test_caller_selection_is_not_mutated(self):
+        box = np.eye(3).reshape(1, 3, 3) * 10
+        positions = np.array([[[0.0, 0.0, 0.0], [1.0, 0.0, 0.0]]])
+        selection = [None, None]
+        compute_rdf(box, positions, np.array([0, 1]), selection)
+        self.assertEqual(selection, [None, None])