renpy/doc/reference/functions/renpy.watch
From Ren'Py Visual Novel Engine
renpy.watch
| Function: | renpy.watch | (expression, style='default', **properties): |
This watches the given python expression, by displaying it in the upper-left corner of the screen (although position properties can change that). The expression should always be defined, never throwing an exception.
A watch will not persist through a save or restart.
Example
init: $ renpy.watch("renpy.get_filename_line()")
