Feat: Add n_initial_contacts attribute and document Contacts class attributes

package/CHANGELOG

@@ -27,6 +27,8 @@ Fixes
  * Fix deploy action to use the correct version of the pypi upload action.
 
 Enhancements
+ * Documented the r0 attribute in the `Contacts` class and added the 
+   `n_initial_contacts` attribute, with documentation. (Issue #2604, PR #4415)
 
 Changes
  * As per NEP29, the minimum version of numpy has been raised to 1.23.

package/MDAnalysis/analysis/contacts.py

@@ -405,6 +405,13 @@ def __init__(self, u, select, refgroup, method="hard_cut", radius=4.5,
              Show detailed progress of the calculation if set to ``True``; the
              default is ``False``.
 
+        Attributes
+        ----------
+        n_initial_contacts : int
+             Total number of initial contacts.
+        r0 : list[numpy.ndarray]
+             List of distance arrays between reference groups.
+
         Notes
         -----
 
@@ -455,6 +462,9 @@ def __init__(self, u, select, refgroup, method="hard_cut", radius=4.5,
                                                 box=self._get_box(refA.universe)))
                 self.initial_contacts.append(contact_matrix(self.r0[-1], radius))
 
+        self.n_initial_contacts = self.initial_contacts[0].sum()
+
+
     @staticmethod
     def _get_atomgroup(u, sel):
         select_error_message = ("selection must be either string or a "

testsuite/MDAnalysisTests/analysis/test_contacts.py

@@ -375,6 +375,16 @@ def test_warn_deprecated_attr(self, universe):
         with pytest.warns(DeprecationWarning, match=wmsg):
             assert_equal(CA1.timeseries, CA1.results.timeseries)
 
+    @pytest.mark.parametrize("datafiles, expected", [((PSF, DCD), 0),
+                                                     ([TPR, XTC], 41814)])
+    def test_n_initial_contacts(self, datafiles, expected):
+        """Test for n_initial_contacts attribute"""
+        u = mda.Universe(*datafiles)
+        select = ('protein', 'not protein')
+        refgroup = (u.select_atoms('protein'), u.select_atoms('not protein'))
+
+        r = contacts.Contacts(u, select=select, refgroup=refgroup)
+        assert_equal(r.n_initial_contacts, expected)
 
 def test_q1q2():
     u = mda.Universe(PSF, DCD)