pairs by type (#4022)

Description of changes:
- add a ``types`` argument to ``system.cell_system.get_pairs()`` to look only for pairs of particles with the given types
- Interface change: removed the ``_`` from the end of ``get_pairs_()``

src/python/espressomd/cellsystem.pyx

@@ -143,9 +143,16 @@ cdef class CellSystem:
         This function needs to be separated from ``self.get_pairs()`` because ``cdef``
         cannot be inside an ``else`` block
         """
+        if hasattr(types, "__getitem__"):
+            if len(types) == 1:
+                check_type_or_throw_except(
+                    types[0], 1, int, 'types must be a list of int')
+            else:
+                check_type_or_throw_except(
+                    types, len(types), int, 'types must be a list of int')
+        else:
+            raise ValueError('types must be iterable')
 
-        check_type_or_throw_except(
-            types, len(types), int, 'types must be a list of int')
         cdef vector[int] types_c
         for type in types:
             types_c.push_back(type)

testsuite/python/pairs.py

@@ -79,7 +79,13 @@ def check_pairs(self):
         epairs_by_type = self.expected_pairs_with_types(self.s.periodicity)
         self.assertSetEqual(set(pairs_by_type), set(epairs_by_type))
 
-    def check_exception(self):
+    def test_input_exceptions(self):
+        with self.assertRaises(ValueError):
+            self.s.cell_system.get_pairs(0.1, types=3)
+        # check no exception for list of length 1
+        self.s.cell_system.get_pairs(0.1, types=[3])
+
+    def check_range_exception(self):
         with self.assertRaises(Exception):
             self.s.cell_system.get_pairs(3.)
 
@@ -103,13 +109,13 @@ def test_dd(self):
         self.s.cell_system.set_domain_decomposition()
         self.s.periodicity = [1, 1, 1]
         self.run_and_check()
-        self.check_exception()
+        self.check_range_exception()
 
     def test_dd_partial_z(self):
         self.s.cell_system.set_domain_decomposition()
         self.s.periodicity = [1, 1, 0]
         self.run_and_check()
-        self.check_exception()
+        self.check_range_exception()
 
 
 if __name__ == "__main__":