fixed system.part.pairs() (#4628)

Fixes #4622

Description of changes:
- the `System.part.pairs()` method now returns the correct particle pairs when particle ids aren't both contiguous and starting from 0

src/python/espressomd/particle_data.py

@@ -26,6 +26,7 @@
 from .utils import check_type_or_throw_except
 from .code_features import assert_features, has_features
 from .script_interface import script_interface_register, ScriptInterfaceHelper
+import itertools
 
 
 @script_interface_register
@@ -1176,10 +1177,9 @@ def pairs(self):
         """
 
         ids = self.call_method("get_particle_ids")
-
-        for i in ids:
-            for j in ids[i + 1:]:
-                yield (self.by_id(i), self.by_id(j))
+        id_pairs = itertools.combinations(ids, 2)
+        for id_pair in id_pairs:
+            yield (self.by_id(id_pair[0]), self.by_id(id_pair[1]))
 
     def select(self, *args, **kwargs):
         """

testsuite/python/pairs.py

@@ -122,6 +122,23 @@ def test_dd_partial_z(self):
         self.run_and_check()
         self.check_range_exception()
 
+    def test_non_consecutive(self):
+        self.system.part.clear()
+        self.system.part.add(id=100, pos=(0.1, 0.5, 0.5))
+        self.system.part.add(id=200, pos=(0.2, 0.5, 0.5))
+        self.system.part.add(id=1, pos=(0.3, 0.5, 0.5))
+        self.system.part.add(id=2, pos=(0.4, 0.5, 0.5))
+
+        self.system.integrator.run(0)
+
+        pairs = self.system.part.pairs()
+        pairs_ids = []
+        for pair in pairs:
+            pairs_ids.append(tuple(sorted([pair[0].id, pair[1].id])))
+        expected_pairs = [(1, 2), (1, 100), (2, 200),
+                          (100, 200), (2, 100), (1, 200)]
+        self.assertSetEqual(set(pairs_ids), set(expected_pairs))
+
 
 if __name__ == "__main__":
     ut.main()