| Writing new tests | 
 | ================= | 
 |  | 
 | Precaution | 
 | ---------- | 
 |  | 
 |     New tests should always use only one Tk window at once, like all the | 
 |     current tests do. This means that you have to destroy the current window | 
 |     before creating another one, and clean up after the test. The motivation | 
 |     behind this is that some tests may depend on having its window focused | 
 |     while it is running to work properly, and it may be hard to force focus | 
 |     on your window across platforms (right now only test_traversal at | 
 |     test_ttk.test_widgets.NotebookTest depends on this). | 
 |  |