Look up parent todo items more efficiently

todo_by_dep_id was quite inefficient due to the linear search, calling
Todo.tag_value() many times to fetch the dependency ID.

Instead, maintain a dictionary that maps a parent ID to a todo item so
it can be looked up more easily.

test/test_todo_list.py

@@ -330,6 +330,7 @@ class TodoListDependencyTester(TopydoTest):
 
         self.assertFalse(from_todo.has_tag('id'))
         self.assertFalse(to_todo.has_tag('p'))
+        self.assertFalse(self.todolist.todo_by_dep_id('2'))
 
     def test_remove_dep2(self):
         old = str(self.todolist)
@@ -338,6 +339,9 @@ class TodoListDependencyTester(TopydoTest):
         self.todolist.remove_dependency(from_todo, to_todo)
 
         self.assertEqual(str(self.todolist), old)
+        self.assertTrue(self.todolist.todo_by_dep_id('1'))
+        self.assertTrue(self.todolist.todo_by_dep_id('2'))
+        self.assertTrue(self.todolist.todo_by_dep_id('3'))
 
     def test_remove_dep3(self):
         """ Try to remove non-existing dependency. """
@@ -347,6 +351,9 @@ class TodoListDependencyTester(TopydoTest):
         self.todolist.remove_dependency(from_todo, to_todo)
 
         self.assertEqual(str(self.todolist), old)
+        self.assertTrue(self.todolist.todo_by_dep_id('1'))
+        self.assertTrue(self.todolist.todo_by_dep_id('2'))
+        self.assertTrue(self.todolist.todo_by_dep_id('3'))
 
     def test_remove_todo_check_children(self):
         todo = self.todolist.todo(2)
@@ -359,6 +366,7 @@ class TodoListDependencyTester(TopydoTest):
         todo = self.todolist.todo(3)
         self.todolist.delete(todo)
         self.assertFalse(todo.has_tag('p', '2'))
+        self.assertFalse(self.todolist.todo_by_dep_id('2'))
 
         todo = self.todolist.todo(1)
         children = self.todolist.children(todo)
@@ -427,19 +435,7 @@ class TodoListCleanDependencyTester(TopydoTest):
         self.todolist.clean_dependencies()
 
         self.assertFalse(self.todolist.todo(1).has_tag('id'))
-
-    def test_clean_dependencies4(self):
-        """ Clean p: items when siblings are still connected to parent. """
-        self.todolist.add("Foo id:1")
-        self.todolist.add("Bar p:1")
-        self.todolist.add("Baz p:1 id:2")
-        self.todolist.add("Buzz p:2 p:1")
-
-        self.todolist.clean_dependencies()
-
-        self.assertFalse(self.todolist.todo(4).has_tag('p', '1'))
-        self.assertTrue(self.todolist.todo(1).has_tag('id', '1'))
-        self.assertTrue(self.todolist.todo(2).has_tag('p', '1'))
+        self.assertFalse(self.todolist.todo_by_dep_id('1'))
 
 
 if __name__ == '__main__':

topydo/lib/TodoList.py

@@ -39,6 +39,7 @@ class TodoList(TodoListBase):
         """
         # initialize these first because the constructor calls add_list
         self._tododict = {}  # hash(todo) to todo lookup
+        self._parentdict = {}  # dependency id => parent todo
         self._depgraph = DirectedGraph()
 
         super().__init__(p_todostrings)
@@ -49,9 +50,10 @@ class TodoList(TodoListBase):
         There is only one such task, the behavior is undefined when a todo item
         has more than one id tag.
         """
-        hits = [t for t in self._todos if t.tag_value('id') == p_dep_id]
-
-        return hits[0] if len(hits) else None
+        try:
+            return self._parentdict[p_dep_id]
+        except KeyError:
+            return None
 
     def _maintain_dep_graph(self, p_todo):
         """
@@ -61,6 +63,7 @@ class TodoList(TodoListBase):
         dep_id = p_todo.tag_value('id')
         # maintain dependency graph
         if dep_id:
+            self._parentdict[dep_id] = p_todo
             self._depgraph.add_node(hash(p_todo))
 
             # connect all tasks we have in memory so far that refer to this
@@ -175,6 +178,7 @@ class TodoList(TodoListBase):
 
             if not self.children(p_from_todo, True):
                 p_from_todo.remove_tag('id')
+                del self._parentdict[dep_id]
 
             self.dirty = True
 
@@ -220,6 +224,7 @@ class TodoList(TodoListBase):
                 value = todo.tag_value('id')
                 if not self._depgraph.has_edge_id(value):
                     remove_tag(todo, 'id', value)
+                    del self._parentdict[value]
 
         def clean_orphan_relations():
             """