Instantiate ListFormatParser only once for each format string

There is no need to instantiate the parser for each widget since the
expression remains the same. Should save some CPU cycles for each screen
update, in my case having over 600 todo widgets activated.

topydo/ui/TodoWidget.py

@@ -20,6 +20,10 @@ from topydo.ui.Colors import color_map256
 
 import urwid
 
+# pass a None todo list, since we won't use %i or %I here
+PRIO_FORMATTER = ListFormatParser(None, "%{(}p{)}")
+TEXT_FORMATTER = ListFormatParser(None, "%s %k\n%h")
+
 def _markup(p_todo, p_focus):
     """
     Returns an attribute spec for the colors that correspond to the given todo
@@ -47,12 +51,8 @@ class TodoWidget(urwid.WidgetWrap):
         # clients use this to associate this widget with the given todo item
         self.todo = p_todo
 
-        # pass a None todo list, since we won't use %i or %I here
-        prio_formatter = ListFormatParser(None, "%{(}p{)}")
-        text_formatter = ListFormatParser(None, "%s %k\n%h")
-
-        todo_text = text_formatter.parse(p_todo)
-        priority_text = prio_formatter.parse(p_todo)
+        todo_text = TEXT_FORMATTER.parse(p_todo)
+        priority_text = PRIO_FORMATTER.parse(p_todo)
 
         id_widget = urwid.Text(str(p_number), align='right')
         priority_widget = urwid.Text(priority_text)