Fix adding empty set to disjoint sets

algolib/structures/disjoint_sets.py

@@ -66,13 +66,15 @@ def add(self, elements: Iterable[_T], represent: Optional[_T] = None):
             if element in self:
                 raise ValueError(f"Value {element} already present.")
 
-        set_represent = self._represents[represent] if represent is not None else elements_tuple[0]
+        if len(elements_tuple) > 0:
+            set_represent = self._represents[represent] if represent is not None else \
+                elements_tuple[0]
 
-        for element in elements_tuple:
-            self._represents[element] = set_represent
+            for element in elements_tuple:
+                self._represents[element] = set_represent
 
-        if represent is None:
-            self._count += 1
+            if represent is None:
+                self._count += 1
 
         return self
 

tests/structures/test_disjoint_sets.py

@@ -90,6 +90,12 @@ def test__op_iadd__when_empty__then_new_set(self):
 
         assert_that(self.test_object).is_length(1)
 
+    def test__op_iadd__when_empty_new_elements__then_no_changes(self):
+        # when
+        self.test_object += []
+        # then
+        assert_that(self.test_object).is_length(len(self.numbers))
+
     def test__op_iadd__when_new_elements__then_new_set(self):
         # when
         self.test_object += self.absent
@@ -120,6 +126,12 @@ def function(elements):
     # endregion
     # region add
 
+    def test__add__when_empty_new_elements_to_present_represent__then_no_changes(self):
+        # when
+        self.test_object.add([], self.present[0])
+        # then
+        assert_that(self.test_object).is_length(len(self.numbers))
+
     def test__add__when_new_elements_to_present_represent__then_added_to_existing_set(self):
         # given
         represent = self.present[0]