Fix class attributes, instantiate in __init__.

TodoBase.py

@@ -19,10 +19,10 @@ class TodoBase(object):
     in a todo item.
     """
 
-    src = None
-    fields = {}
-
     def __init__(self, p_src):
+        self.src = ""
+        self.fields = {}
+
         self.set_text(p_src)
 
     def tag_value(self, p_key):

TodoFile.py

@@ -8,7 +8,6 @@ class TodoFile(object):
     to.
     """
 
-    path = ""
     def __init__(self, p_path):
         self.path = p_path
 

TodoList.py

@@ -12,13 +12,13 @@ class TodoList(object):
     The list is usually a complete list found in the program's input (e.g. a
     todo.txt file), not an arbitrary set of todo items.
     """
-    _todos = []
 
     def __init__(self, p_todostrings):
         """
         Should be given a list of strings, each element a single todo string.
         The string will be parsed.
         """
+        self._todos = []
         for string in p_todostrings:
             self._todos.append(Todo.Todo(string))