cluster_analysis: fix fractal_dimension missing-dr error and docs (bug-sweep #51)

src/python/espressomd/cluster_analysis.py

@@ -42,7 +42,7 @@ class Cluster(ScriptInterfaceHelper):
     longest_distance()
         Longest distance between any combination of two particles in the cluster
 
-    fractal_dimension(dr=None)
+    fractal_dimension(dr)
         Estimates the cluster's fractal dimension by fitting the number of
         particles :math:`n` in spheres of growing radius around the center of mass
         to :math:`c*r_g^d`, where :math:`r_g` is the radius of gyration of the

src/script_interface/cluster_analysis/Cluster.hpp

@@ -57,7 +57,7 @@ class Cluster : public AutoParameters<Cluster> {
     }
     if (method == "fractal_dimension") {
       auto const [df, mean_sq_residual] =
-          m_cluster->fractal_dimension(get_value<double>(parameters.at("dr")));
+          m_cluster->fractal_dimension(get_value<double>(parameters, "dr"));
       return std::vector<double>({df, mean_sq_residual});
     }
     if (method == "center_of_mass") {

testsuite/python/cluster_analysis.py

@@ -200,6 +200,31 @@ def test_single_cluster_analysis(self):
             df = self.cs.clusters[cid].fractal_dimension(dr=0.001)
             self.assertAlmostEqual(df[0], 2, delta=0.08)
 
+    def test_fractal_dimension_requires_dr(self):
+        # ``dr`` is a mandatory parameter (the histogram bin increment). The
+        # missing-parameter check happens during script-interface argument
+        # evaluation, before the GSL-guarded core function body, so this test
+        # does not require the GSL feature.
+        for x in np.arange(-0.2, 0.21, 0.01):
+            self.system.part.add(pos=(x, 1.1 * x, 1.2 * x))
+        dc = espressomd.pair_criteria.DistanceCriterion(cut_off=0.13)
+        self.cs.pair_criterion = dc
+        self.cs.run_for_all_pairs()
+        self.assertEqual(len(self.cs.clusters), 1)
+        cluster = list(self.cs.clusters)[0][1]
+
+        # Omitting ``dr`` entirely must raise the standard friendly
+        # "missing parameter" error (not an opaque ``unordered_map::at``).
+        with self.assertRaisesRegex(
+                RuntimeError, "Parameter 'dr' is missing"):
+            cluster.fractal_dimension()
+        # Passing ``dr=None`` (which the old docstring advertised as the
+        # default) must raise a clear conversion error, since ``dr`` has no
+        # meaningful default and ``None`` is not a valid float.
+        with self.assertRaisesRegex(
+                RuntimeError, "not convertible to 'double'"):
+            cluster.fractal_dimension(dr=None)
+
     def test_analysis_for_bonded_particles(self):
         self.set_two_clusters()
         # Run cluster analysis