Eliminate possibility for multiple todos having the same id: value.

When there is a todo in the list with an id: tag but no child todos,
that value could be reused when creating a new dependency between two
(unrelated) items.

This is because the value of the dangling parent todo is not present in
the dependency graph, but the function generating a new ID depended on
that.

Instead, simply iterate over all available todo items to check whether
the proposed value already exists somewhere.

test/TodoListTest.py

@@ -238,6 +238,7 @@ class TodoListDependencyTester(TopydoTest):
         self.todolist.add("Something with +Project")
         self.todolist.add("Another one with +Project")
         self.todolist.add("Todo with +AnotherProject")
+        self.todolist.add("Todo without children id:3")
 
     def test_check_dep(self):
         children = self.todolist.children(self.todolist.todo(1))
@@ -268,8 +269,8 @@ class TodoListDependencyTester(TopydoTest):
         todo5 = self.todolist.todo(5)
         self.todolist.add_dependency(todo5, todo4)
 
-        self.assertTrue(todo5.has_tag('id', '3'))
-        self.assertTrue(todo4.has_tag('p', '3'))
+        self.assertTrue(todo5.has_tag('id', '4'))
+        self.assertTrue(todo4.has_tag('p', '4'))
 
     def test_add_dep2(self):
         """
@@ -284,8 +285,8 @@ class TodoListDependencyTester(TopydoTest):
         self.todolist.add_dependency(todo5, todo4)
         self.todolist.add_dependency(todo4, todo1)
 
-        self.assertTrue(todo4.has_tag('id', '4'))
-        self.assertTrue(todo1.has_tag('p', '4'))
+        self.assertTrue(todo4.has_tag('id', '5'))
+        self.assertTrue(todo1.has_tag('p', '5'))
 
     def test_add_dep3(self):
         """

topydo/lib/TodoList.py

@@ -115,9 +115,19 @@ class TodoList(TodoListBase):
             Unused means that no task has it as an 'id' value or as a 'p'
             value.
             """
+            def id_exists(p_id):
+                """
+                Returns True if there exists a todo with the given parent
+                ID.
+                """
+                for todo in self._todos:
+                    if todo.has_tag('id', str(p_id)):
+                        return True
+
+                return False
 
             new_id = 1
-            while self._depgraph.has_edge_id(str(new_id)):
+            while id_exists(new_id):
                 new_id += 1
 
             return str(new_id)